Yeah the key is to lock the pulleys, not try and lock it from the timing end, the ends of the cams are very fragile and the lips that make the two outer semi circles the timing gear goes onto will just bend when you try and undo the dephasor bolt if that is what you try and rely on to lock the cams in place.
*edit*
Replied before I saw Fred's reply, so it sounds like even if the pulleys are locked properly you could still get issues.
You DEFINATELY will if they arent though, Ive seen the results of someone trying to rely on just the timing end to hold them in place, cams were ruined.