If you call for an extra blanket you shouldn't leave an extra $1 in the housekeeping envelope--you should tip the guy who brought you the blanket. Or did I misunderstand something? At any rate, I usually don't leave a tip in the room because its too hard to determine who gets it--different people clean on different days, often more than one lady cleans a room, how to split it up? I try to do this--usually there's a card in the room "your room was cleaned today by 'Mary'". I then put an appropriate tip in an envelope for 'Mary' and leave it at the front desk. Just hope she gets it. Or if I see the cleaning lady I try to give the tip directly to her.
I realize they work hard for little money, however, when I'm paying $300+ for a room I really don't think I should have to pay "extra" for basic cleaning, supplies, etc. Isn't that what I'm paying for in the first place? If I make extra requests, such as more towels, clean up kids throw-up, of course, then I leave a tip.