You might get away with no GC before a snapshot but please remember that you do need to do various shutdown work to make the written snapshot have a serious chance of being restartable.
I'm not in a position to provide a list; never tried it so I don't know what would go wrong. I _do_ know that a similar 'neat trick' cost PPS a very large amount of engineer time (and therefore direct money out of my budget as manager of the dept. back then) when so twerp at a Large Oval Logo Company decided that snapshotting in the background every few minutes would be a Good Idea. I can't recall the details any longer - getting too old - but it took _months_ to track down the problem. It was escalated to a total emergency, the GC was blamed, my engineers were blamed, I was blamed, the customer was adamant that they had never, would never, could never have done anything that could have caused it. When we solved the problem and proved it was their code I don't recall anybody even saying thanks.
So all in all I'm not sure I'd recommend doing a regular raw snapshot in the background.
tim