Even if drilled on rappel, on many slab climbs, it's better to have the bolt at some sort of stance or rest where one can stop moving long enough to suck up a bight and clip. At least on 5.10+ and up slab, for me, I find a lot of times a smooth motion is the only thing that keeps me going, and if I stop in the middle of some friction moves, POP. If that stance is (gasp) above the crux, so be it.
Then again I have once or twice put up climbs on lead without bolts due to lack of a drill, and deliberately located the bolts placed later so that when one is cruxing the bolt will be below foot level and the crux hence uncheatable or as the French say, "obligatoire".