yeah, thats called cheating...=) it effectively turns the exercise into a different exercise...specifically called "decline bench press". its especially bad when your ass actually raises off the bench.
its the same deal when you see people doing barbell shoulder press, but their ass is like 2 feet away from the back of the chair and their back is arched further than my bitch's when she's riding me. it effectively turns the exercise into "incline bench press".
you're only cheating yourself...
-- clocky