Sorry for the delay - last night I had ~1/2 a response typed... and then we lost power GRRR!

Most of the time any air or water that gets behind the liner isn't a problem. What IS a problem is if debris gets into the gap, it will show up as a bump or lump behind the liner, and if it's sharp, could poke a hole in the liner.

I always do it from the deck, but you can get in the pool and do it, if that is easier for you.