Checking exhaust system for leaks

I am interested in checking exhaust system for leaks. dddart 66 with 6cyl. exhaust system looks old and original. muffler is a Imco 28. I searched on www and there are guys that are hooking up a vacuum to the exhaust and running it (blowing it back into pipe) and then checking with bubbles sprayed onto pipes etc... My question, is this ok to do? Is there any sort of damage that can occur to the engine by blowing into it from the exhaust?
I also saw a guy letting vacuum cleaner suck out of the exhaust and then go listen to the pipes and hear a hiss?