NO!!!
I do not do this with any of my clocks for the exact reason you mention... I'll know that I actually have 15 minutes to spare, I'll screw around some....and wind up being late anyway. It has never worked for me, or anybody I know who does this, as they all wind up doing the same thing I do.
I'm just a big time waster.... that's why I'm here instead of doing work or schoolwork!!