I always tip $5/night, regardless of whether I'm in a Motel 6 (used to stay in them all the time when I was young because I knew they were dog-friendly), the W, or a Kimpton (where I more often stay for work). The only exception is when I stay at private clubs because a lot of them have an explicit no-tipping policy because the staff are paid about double what staff at a normal hotel are paid.
My general rule of thumb on tipping is to ask two questions:
First, am I getting truly personal service? Cleaning my personal space or cutting my hair, for instance, would definitely qualify Fixing my car, which I drop off at the garage, does not.
Second, is the person giving me the service a "professional"? So a dentist is definitely giving a personal service, but he might well make more than I do, so no tip. Same goes for a plumber or electrician.
After those questions, I also employ a bit of a sliding scale, where my perception of the person's income gets weighed against the clarity of the tipping convention. So for instance, the most obvious example of someone you tip is a waiter. At nice restaurants, however, waiters make incredible money. That doesn't mean I don't tip, though, because the convention is so strong. By contrast, it's clear that there is no universal rule that maids should be tipped. But it's also clear that maids make very little money, so I have no qualms at all about giving a tip.