I've had to use it once or twice, but not very often. Basically, your skin code comes after all the default site scheme stuff, so will override it by CSS inheritance rules if they are equally specific, or if yours is more specific. If yours is less specific than the original rule, you'll need to use !important. However, if you're having to use it a lot, it basically means either there's something wrong with our code, or something wrong with yours, so it's worth sending in a support request with the code for your skin, or post it here and someone can have a look.