Dipping usually refers to using some sort of metal cleaner, in other words an acid. Basically what happens is that the acid strips away part of the coin's surface, usually the toned part, leaving behind a bright surface. Since this alters the surface of the coin it is cleaning and should be noted (for modern coins at least) by those of high ethics.
Acetone is unreactive with silver and gold. If you dip an untreated silver coin in acetone nothing should happen. If you have a piece of tape on your coin, acetone should dissolve it. An acetone dip for silver and gold should not impact the surface of the coin and thus isn't technically cleaning the coin, it may remove some surface contminents.
Ammonia is better for silver toning than acidic dips, it mainly reacts with the oxide so it is still technically cleaning but it is much less harsh than the acidic dips.
Copper is much more reactive and should generally be left alone.
Has any read the coin chemistry book?
Brent Krueger