Honestly at the end of the day it is tactical. PLaying MM? Kill his killers, they will do you less damage in the future, in fact kill anything that is TV efficient. Playing in a league? Leave him in a pristine condition if he can't threaten you because you need him at his best to play your opponents. Conversly if he is ahead of you and you need him to lose a few games, cripple the blighter. I played one chap in tier 2 of the OCC last season, we were both getting promoted, he is a better player than me and so I set out to cripple his team once luck put me safely in the lead with as many fouls as possible. He still won his first game in div 1

Anyone who says oh I always do it, why don't you always leap your wardancer every turn to make it like she is dancing? Fouling is a decision like any other in bloodbowl, there should be a reason behind it or you are just messing around and when I game takes more than an hour, bit annoying for your opponent.