Like Amun say, you can use BR at final stage,during Pelline, when all minions are dead.
You don't need to use it before, others stages are easy.
Just remeber to announce that you gonna BR, to be sure that all people will be aware that they're gonna do some higher damage ( for exemple, Feoh like to usedouble cast, Othell clones, etc).
Of course you do it after all minions are dead in stage 7, and only the boss remains. More importantly, if you have a summoner in group, do not do it until he changes to cats.