[The sync voltage limit on a Cactus V5 is 300V][1]. 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][2], the technical rep for Canon. The 6V limit was only for the 300D and earlier Canon dSLRs.


  [1]: http://www.gadgetinfinity.com/blog/2011/01/1164/
  [2]: http://digitaljournalist.org/issue0712/tech-tips.html