[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, answering a question in a 2007 tech tips article for the digitaljournalist.org:

> **I recently posted a question regarding the safe maximum sync voltage for an EOS 30D on http://www.openphotographyforums.com/. A reply led
> me to an article called Tech Tips answering a number of Canon-related
> FAQ. You addressed the safe sync voltage for a number of models,
> including the 20D, but I was wondering where I might be able to find
> published data on the safe sync voltages for the entire range of Canon
> cameras (or maybe just the 30D, as that's the body I'm using now).**
> 
> It's likely you'll never see an official list of all Canon SLRs
> according to this specification, because Canon Inc. (our parent
> company in Japan) simply doesn't do things like that. I've been with
> Canon USA since 1982, so I'm in a pretty good position to know Canon
> Inc.'s habits. However, I'll be happy to provide you with my
> unofficial list:
> 
> **Canon Digital SLRs safe for TCV up to 250 volts:** EOS-1D Mark II N, EOS-1D Mark II, EOS-1Ds Mark II, EOS-1D, EOS-1Ds EOS 30D, 20D, 5D EOS
> Digital Rebel XTi, XT (400D/350D) EOS D6000/D2000, Kodak DCS560/DCS520
> (circa 1998) EOS-DCS series (circa 1995)
> 
> **Canon Digital SLRs safe for TCV up to 6 volts:** EOS 10D, D60, D30 EOS Digital Rebel (300D)
> 
> **Canon 35mm SLRs safe for TCV up to 250 volts:** EOS-1V, EOS-1N, EOS-1, EOS 3
> 
> **Canon 35mm and IX240 SLRs safe for TCV up to 6 volts:** EOS 650, 620, 630, RT EOS 850, 750, 700 EOS Rebel Series EOS Elan Series EOS
> 10s, A2E, A2 EOS IX, IX Lite T90
> 
> Canon SLRs released earlier than the T90 did not have TTL flash
> circuits, and comprehensive information on safe TCV levels is not
> available.
> 
> The trigger circuit voltage (TCV) rating for any EOS SLR is the same
> on the hot shoe as it is on the PC terminal (if the camera has one),
> but the acceptable TCV level varies according to the camera model.
> Incidentally, the main reason for the difference is the way the X-sync
> signal is generated. With the 250V cameras, the X-sync signal is
> generated electronically. With the 6V cameras, the X-sync signal is
> generated mechanically. There are no guarantees, but going forward I
> anticipate that most if not all future EOS SLRs will be safe for TCV
> up to 250 volts.

  [1]: http://www.gadgetinfinity.com/blog/2011/01/1164/
  [2]: http://digitaljournalist.org/issue0703/tech-tips.html