Is it safe to disassemble an unpowered PC power supply?
Assume: Typical PC power supply = AC mains powered.

There is a short term risk of electric shock from capacitors - principally the two large capacitors used in a "half bridge' arrangement in many PC supplies. These are arranged in such a manner that the supply may be switched between nominal 230 VAC and 110 VAC easily. These capacitors will happily kill you if you let them.

Discharge time for main capacitors is usually seconds with a supply designed to do this.
 It can be minutes.
 Worst case you MIGHT expect hours.
 You'd be most unlikely to find them alive after a day. [If you do, note the brand and buy them in future].

In any case, when dealing with large capacitors that have had high voltage on them I will short them with a piece of wire or screwdriver tip etc. 

Note that some capacitors will "recharge themselves" partially due to dielectric absorption. This can take place over minutes and can be exciting. Not usually a major issue but be aware of it. 

"The book" will say you should use a resistors, insulated probes and safety glasses.
 Using an insulated screwdriver tip, turning your head, shutting your eyes and flinching will usually allow you to "safely" discharge "rather large"  capacitors with HV still on them (as long as the source of the HV has been removed) but it can be hard on the nerves, you can get spattered with bits of screwdriver tip, it can make it hard to undo screws in future and the insurance company may refuse to pay your widow. ie use common sense. (Long long ago I did this with 1000 Volts on a cap and the supply still connected "not quite on purpose" - definitely not recommended. The screwdriver tip needed re-grinding :-) ) 

Other caps of note are the smaller X caps across phase - neutral and the Y caps from either mains lead to ground. These are usually high quality non polarised and MAY happily hold charge for a long time - test discharging them is a good idea. These are usually not large enough to kill (YMMV) but they can hurt badly, and reflexively jerking your hand onto something sharp, hot or live is a risk.