I was shouted at by the CFI when I was a student pilot, for turning the prop backwards. He drew me a picture of the vac pump and explained what would happen. Lesson not forgotten!
If the mags aren't grounded, nasty things can happen whichever way you turn the prop. Always treat it as "live"!
Our checklist says the prop must be pulled through four compressions before the first flight of the day, to confirm no hydraulic lock.