Yes, you can add a buffer. I find that 1 minute is adequate. Chopping off the beginning or end of a program is a rare but annoying occurance, so adding a little buffer is recommended.
It may become a problem if you're tieing-up all 3 tuners. I seem to have noticed that if you're recording back-to-back programs on the same channel, the buffer for the end of the first program will default to zero, and the buffer for the beginning of the second program will default to zero. This prevents two tuners being necessary for a two-hour block of time on the same channel, for instance. The two events will still be listed as their respective programs on the DVR recordings list. This is just an observation. I may be wrong, but I don't recall changing my buffers, although I noticed it happened while recording NBC's "Heroes" followed by "Studio 60 on the Sunset Strip".