I'd say you should use them until you have mostly lv 2 where they count. The power boost they will provide will let you win more reward in various places (crit will help in fox/coin trial, all the useful stat will help you in pvp where sometimes a win will carry extra reward for one reason or another), considering that lv 1&2 boost are fairly significant, the reward you get from those various places during the potential waiting period will also be potentially significant. The value of rebate you get from those rebate event might not actually match that.

That said, once you have gotten a good set of lv 2s, it hits serious diminishing return and thus won't give you too much of a boost, waiting for rebate could become a far better option.