The Tavern

Crypt is notorious for being a pretty chill place to bide time/gain xp. There are some dangerous monsters (alich and curse skull), but even considering these rare threats crypt 1-2 feels roughly as dangerous as V 1-4. It is definitely safer than depths, and you can clear it after depths if you want. Of course, crypt has no rune (tomb doesn't count) and is therefore optional. Most characters are strong enough/have enough items to take on zot safely even without doing crypt.

Is the marginal risk incurred by exploring crypt worth the marginal boost to xp/character power that clearing the branch provides?

Alternately, is the safest unexplored place to go always the least dangerous unexplored place?

I usually clear it before depths, after elf

You need to add two or three Greater Mummies, and possibly Khufu to the list of dangerous monsters. They can be a nasty surprise, if encountered at the wrong moment.
I like the treasure in Crypt 3, which often fills equipment or spell gaps on my character.

I forgot to answer the question It depends on character. For instance, it is a cakewalk for any character with Dispel Undead but can be a suicide for NaBe without scrolls of blinking, any EV-based character with low MR and weak melee (KoEn) or an IE/VM because many monsters resist cold/poison.
If I don't have rC+ and rN+, I consider skipping Crypt too. If I have just one of them, I still usually clear it because Crypt 3 often has artefact items with rN+ (and often with rN++). Having no SInv can be dangerous too (vampire knight, (ancient) lich, shadow)

As far as winning the game goes, as a human/elf/mummy, you're better off not doing crypt or elf, because by the time you can enter those branches, the main threat to your character is lapses in attention, so it's better to do as few levels as possible. Other good levels to skip for this reason include Zot:1-4 and profane halls.

A hypothetical perfect player playing online probably still wouldn't do crypt or elf, because if there's anything that could cause an unavoidable death at that point, it's a player ghost with 8d21 fire storm, and the fewer levels you do the less likely you are to run into that ghost.

What is 'profane halls'?

This particular Depths layout

http://i60.tinypic.com/jz87lh.jpg

Personally, I follow the minimum number of floors rule suggested above, but primarily because I feel it is poor style to do additional floors for experience and items that you don't need. I would only do more floors if I had made some major mistake in skill allocation or had exceptionally bad item generation.

I don't think clearing crypt makes a significant difference in your odds of winning in the vast majority of games. If anything the difference is slightly negative most of the time.

I am not sure that it is easy to prove that diving is good. I was surprised when a spectator asked me how many runes I have (I was clearing Depths:5 with XL 27 character and had just 2 runes of course). I guess many players don't even reach XL 27 in a 3 rune game because they dive later floors and avoid Elf/Crypt completely. Both Vaults:5 and Zot:5 is much easier when you are XL 27 character...

Edit. Also I remember I killed one ghost in Zot who had fire storm. Fire storm is less scary when you are XL 27 character too so it is not obvious to me which approach is better.

Edit2. I guess diving works better for characters who already reached their full power, for example, most Berserkers already have both melee and ranged weapon at min delay long before Zot and of course extra XP is not going to help them that much. Now compare that to a typical hybrid who is still working on making OoD or Death's Channel castable while Fighting and Armour/Dodging are far from perfect too.

Most of my wins in the last year are hellcrawl (with upstairs) and I can say those almost always end with xl 25 or less. Looking at old dcss games, I usually hit 27 in spite of avoiding additional branches and diving zot. The only game I see where I hit a really low xl at the end, in line with hellcrawl games, was a character that dived both depths and zot. Diving both is definitely not best practice, but it is excellent style in my opinion.

Skipping depths floors works fine with characters with stronger endgames in terms of experience and loot. I think of Ash and Gozag characters here. I think strong god abilities, like Zin's, also make it reasonably safe.

This is odd reasoning. Crypt is not dangerous, so it you skip it to avoid lapses in attention. Profane Halls and zot 1-4 are dangerous, so you skip them to avoid attention lapses. These two statements seem to be pointing in different directions.

I frequently make a choice at some point in the game that says "I am ready to win now with my current gear and amount of XP. Similarly, I decide I want to win now as opposed to get more runes, die in a zig, become stronger, or just murder everything with my strong character." With this choice, I frequently just dive whatever is left in the game. Effectively, I may get to a similar place in terms of skipping crypt and zot and such, but go about it with different reasons.

I rarely do crypt for xp or loot. I frequently do elf, esp if I'm looking for a specific spell or a staff.

I think the implication is that zot:1-4 and profane halls aren't dangerous.

If you re-read my post you might notice that I said nothing about the danger of any of those levels.

Well I'm saying the different areas have profoundly different effects on your attention, and this is directly related to how dangerous they are. Profane halls are much less likely to kill you because your attention lapses. They are also much more likely to kill you than crypt.

To me the defining features of profane halls and Zot aren't that they are implicitly scary, but that they take a really long time to do (compared to, say, the average Depths level), therefore I'm more likely to stop paying attention.

Edit: nevermind!

It's a probably a bad idea to clear that one skull vault because Jory, but I always do it anyway