I manage my optician's computers. As a bonus I get to pick out new glasses when I see something that I like.
When it comes to different pairs with about the same specifications it's perfectly safe to alternate between them. If there's a drastic difference between them it can be dangerous, but if you can see normally with them it's usually ok.
At the moment I got 4 regular pairs, 1 cheap workout piece (1/10'th the price of some of the others, reground glasses from a previous pair I damaged.) and 2 sunglasses with strenght. Favourite pair is a red Le Tanneur designer piece.
Shapes & weight is what you'll notice most when switching between pairs. The ground glass won't matter that much most of the time. I got everything from top of the line essilor photosensitive high-index to cheaper, plain optical glass. The multicoating on the pricey essilor glass is nice, slightly better antireflective effect than the others, and easier to clean.
Things to watch out for when buying new glasses: Going for cheap plastic lenses is ok, just make sure it's hardened, otherwise it scratches way too easily. Antireflex coating is important if reflexes annoys you. If you have high strenghts, get high-index glass, makes it lighter, slimmer and actually stronger, with no negative side effects. Get big enough pads, your nose will thank you, espeicially with heavier frames.
Glass makers which are good: Zeiss, Essilor, Rodenstock, Hoya. Nonames can be just as good, but if you want neat stuff like very high index glass or photosensitity consider paying a bit extra. In the long run you will always save by getting hardened glass, as otherwise it just takes one mistake to get bad scratches.