I only play Roto, and while I try not to make a habit of spot-starting, it still makes sense to play your matchups, and if there's a favorable matchup of a pitcher on the WW, then I'll definitely pick him up. However, the only pitchers I'll generally spot-start are ones that:
1. are facing a lineup that is both bad and cold.
(I love starting pitchers against Colorado away from Coors, for example, but since they're hitting pretty well tonight, I'm not quite as confident to do that for tomorrow. And when the Royals are hitting poorly, I like using them also, but I've noticed that they can get hot every so often.)
2. have a relatively low walk rate.
(I figure that a pitcher who doesn't walk too many guys has less of a chance of blowing up.)