I never go all the way down...
on incline presses. I stop about 4 inches from my chest when the contraction is maximized. Dropping lower than that on inclines will transfer the forces towards the shoulder and slacken the upper pec. I like the isolation of the upper chest on inclines. Put your hand over someones upper chest when they are doing inclines and you'll actually feel the point of peak contraction when the bar is 3-5 inches from their chest,drop it below that and the upper pec temporarily relaxes and that's what I try to aviod plus it does save wear and tear on your rotator cuff muscles.
I do drop the whole way for flat bench,but I think of flat bench as a more of a power exercise(not an isolation exercise) for pecs-tris and even some trap recrutment.