I do the same thing. I think it’s also a good idea to check every disk under a bright light before burning it, expecially if you’re burning Audio CDs (it might happen to find some interesting things like fingerprints, oil spots, etc…). Of course dirty disks should be used only for second choice burnings (music to listen to in the car for instance) - even if you try to clean them you wouldn’t be able to get good results.
In my experience, such spots as those mentioned by gcmospeada are typical of low quality media (particles of dust aren’t so big) and possibly they are not related to recording speed. Once (in 1999!) I bought a small batch of King Pro Mediatek CD-Rs (plain silver label, w/o writings) and after I noticed they had several such spots (appearing AFTER they’d been burned) I tried to burn them at different speeds, but w/o getting better results. In any case, now, after 6 years (I just checked one), they still work :-)))