As PYW said, acetone isn't much good for getting rid of "dirt"; it gets rid of "grime" and "sticky stuff" - paint, old stickytape, PVC residue, that sort of thing. Metal detector finds and uncleaned ancient coins probably aren't going to be affected at all by acetone. It won't hurt them, however, and leaving coins in acetone to soak won't hurt either, but it will rarely help; if the "dirt" doesn't start to come off after 30 seconds in the acetone, leaving it in there for longer isn't going to make any difference. Whatever the "dirt" is, it isn't soluble in acetone.
The acetone will, however, come in handy if you decide to treat those crusty coins with olive oil or some similar oil; once you're done, you'll want to be getting all the oil off, and acetone is excellent at doing just this.
And, as has been said, acetone isn't entirely safe. It's more flammable than petrol, so no smoking, fires or heat sources nearby. If you get it on your fingers, it'll suck the oil right out of your pores, leaving them dry (and stained white from the oils left behind after the acetone evaporated). It's not particularly toxic unless you drink it, but the vapours can cause dizziness, so use it in a well ventilated area. Don't use it on a painted or polished wood floor or surface, unless you were planning on re-varnishing or re-painting it sometime soon.
And keep it away from plastic, especially polystyrene; a tiny splash of acetone can depolymerize a surprisingly large block of polystyrene foam (a fact which chemistry teachers and demonstrators know very well), turning it into a little puddle of goo. Use only glass, metal or ceramic jars and containers.
