I would only open a G1 in a box/on a card if their packaging was damaged or was damaging them. I'm not going to judge someone for opening a MIB/MOC G1 unless like they bait it or otherwise destroy/harm it or the packaging was in amazing condition. I'm not gonna yell at them but I will cry behind the screen.
However, bagged ponies? Open them all!!! In my opinion, bags are hideous and keeping bagged ponies is dumb. Boxes and cards have beautiful art and the ponies are kept in one position (unless the box/card is damaged), while in bags they roll around and make a mess of their hair. I opened a G1 November Chrysanthemum. She's amazing and I loved the experience, and I will treasure her forever (yes, even if I stop collecting there are a few ponies I WILL keep forever). However her hair is a mess from being in a bag for 30 years and her card thing left marks on her (as in it damaged her).
As for G2s I feel pretty much the same. I want to buy a G2 Sundance/Sunsparkle I don't have and open her. :D Obviously I'll try and look for one with a bad bubble, but if its cheap I'm not gonna care.
Slight edit OL1