All lenses have dust inside them. But they'll never show on your pictures. Even scary amounts of mold only show up as a reduction in contrast and brightness.
It's much the same with dirt and even fingerprints on the surfaces of the outermost elements. It's got to be really, really, really, really bad before it shows up.
You know how a supertelephoto can shoot through a chain link fence as if it were glass and it never even shows up? Well, stuff even on a slow wide-angle lens is geometrically comparable. It's so out of focus that that the image of that lone tiny dust particle gets smeared over at least half the frame.
There are exceptions, such as when shooting macro at such short distances that the front lens element is practically on top of the subject, but it's hardly worth mentioning.
My advice? Use a lens hood, always. Don't use filters except for their creative effect, with two exceptions: for environmental sealing on L lenses that require it when you're shooting in a dust storm or in ocean spray or the like; and for traditional protection when you should be wearing eye protction yourself (such as gravel being kicked up by horse hooves at a rodeo). And even then, get the best filter you can and treat it as disposable. Otherwise, keep your fingers off the lens, and only clean them (blower, then brush, then Pec Pad with a drop of Eclipse) when it really bothers you. Cleaning isn't the best thing in the world for the coatings, after all....