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You should always have a UV filter on every lens at all times. Always. A good (multicoated) filter won't degrade image quality, and it protects your lens. Even if you've got a lens hood on to protect from impacts, it'll protect from dust, sand, etc.

I can't stress this enough, mostly because my 30D just took it's first big tumble the other day, and the UV filter saved the lens. This is the third or fourth time I've had that happen. (The previous times were on other bodies).

The old argument against it is that you're shooting through a cheap piece of glass. Be assured that if all the glass in your lens was made to the same demanding specifications as the glass in a quality UV filter, it'd probably cost into the thousands, even for a kit lens. The lens may be more expensive than a filter, but divide that cost by all the pieces of glass and all the moving parts and all the mechanical equipment and all the engineering that goes into them, and you end up with relatively cheap glass.