I think it really depends on the character design, lore, story, etc. I also think the frame size need to match the face, as well as the design match the character. And it should NEVER go with the 'they are hotter without glasses' option. Take a page from Gilmore Girls when Lane goes without her glasses for a bit, and then Zack and Ms. Kim talk about how much better they look on her.
I think it really depends on the character for me. sometimes glasses get unnecessarily added to characters that would look better without, and sometimes it just doesn't match their personality