I often get asked about the little details that go along with painting and many of the questions fall into the "what would you do" category, which usually means, would you paint that. Quite often my answer is yes. I believe that a really good paint job means you paint everything, but this isn't always the case, so I've decided to compile a handy little cheat list for you.
Switch plates. No I wouldn't paint them, paint doesn't stick to that plastic well. BUT I might go buy new ones that match the room a little better. A side note - don't tape around switch plates, remove them and paint under them.
Air Vents/Ducts. Yes, I would definitely paint them. They're ugly and if you don't paint them they stand out as a feature of the room. Make them invisible with a coat or two of paint.
Behind Mirrors. Yes, whenever possible, I remove mirrors and paint behind them. You never know if they'll be replaced and you get a cleaner finish.
Closets. No. I generally don't paint closets and if I'm asked to do them (like a large walk in closet) I like to paint them a lighter color or a neutral color. Dramatic colors (and sometimes even subtle ones) can alter the appearance of clothes.
Ceilings. This totally depends on the situation and the look you're trying to achieve. But if you have a tray ceiling or one with enhancement lights then definitely play up this feature and paint the ceiling.
These are just a few situations and my suggestions. Feel free to ask if you have questions about anything else you might want to paint or tape around so you can get the most professional painting finish possible.