After a lot of research I think I would label it an urban myth that flash units "fry" modern cameras. Yes, there may be some exotic units out there or stuff may just malfunction after all those years. And I don't encourage you to try anything out without doing a bit more research. But after reading lots and lots of forums and web sites on this subject I have not found a single case where somebody actually did harm to the camera with one of these.
Its hardly a myth, and has nothing to do with exotic flash units. Older cameras had a electrical contact rather than electronics to trigger the flash. The flash could put out a high voltage kick. No problem accross mechanical contacts. Then came cameras with electronics to trigger the flash unit, and fried electronics became common until people learned not to connect those old units to a camera.
Here is a list of flashes and their voltages. All Canon units are under 6V, so anything higher carries a level of risk.
The Metz CT-45 is believed to put out about 14.8 volts, so it is a risk, but may or may not cause a problem. As you can see, some of the older Metz flash units put out 200 or even 300V, a sure way to fry your camera.
Use a wireless remote to be safe.
Again, I don't encourage anyone to mess around with this without proper confirmation of what the individual unit puts out - and if it may be defective or not. However, I still believe that this has turned into an urban myth to some degree at least. The 5DII is rated at 250V. There may be some very old or uncommon units that exceed that. Per my information from Metz Germany directly none of the units we discussed here fall into that category. A CT-4 or a CT-5 should (!) be safe. During my "research" on this topic I came across one type that was sold under a different brand name for a German mail order store (sold as "Revue") that looked and functioned just like a CT-4 but appears to have a much higher voltage than the normal Metz branded units.
And then again: I've so far never seen any first-hand report of somebody actually frying their 5D/5DII with a flash. Not saying it never happened, not saying that there isn't some risk involved if a number of factors occur at the same time.
And you are right: to be safe any cheap wireless trigger will be a good idea. Or any of the Wein thingies that insulate the camera. Good thing to have also if you ever shoot on a set with older strobes or so.