The sync voltage limit on a Cactus V5 is 300V. As long as your flash doesn't go over that, you should be fine.
In addition, any dRebels that are newer than the 300D have a sync volt limit of 250V on the hotshoe, according to Chuck Westfall, the technical rep for Canon. The 6V limit was only for the 300D and earlier Canon dSLRs.