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, 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
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
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.