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][1], the technical rep for Canon. The 6V limit was only for the 300D and earlier Canon dSLRs.


  [1]: http://digitaljournalist.org/issue0505/westfall.html