I'm on DeFranco's westside/skinny bastards routine. I followed it to the "T" for the first three months, and got good gains. Now its getting a little stale, so I am varying the reps and sets a little more. In my instance, my legs are my weak spot. So I am concentrating just a little more on them, adding deadlifts to some upper body days.
Tweaking works for me, but only on a small basis. I trust the guys who make the routine, and if it doesn't work you'll know after a month anyway.