It depends on the fish really. I keep my live bearer tank at 7.2-7.6 and they thrive. Somefish, like cichlids will not do well at that level but they can survive if I remember right.
I say leave things alone, you obviously have a grasp on things and the snails are thriving so you are doing something right. It sounds more like a fluke than a serious problem where something spiked all the sudden and it was never caught on a test or just one of those things.
I would say from all you have posted that the one batch that didn't survive well wasn't all that healthy when you got them. Don't fix it if it isn't broken.
The only reason to raise the pH is if you are seeing shell erosion on your snails. Click HERE
to see some examples of erosion.
I have one snail out of 7 that was seriously effected by a weak shell and erosion. Why it was just this one I have no idea but it was very serious. The snail has survived and is thriving now but the damage to the shell was very severe and obvious.