So far two of my characters have glasses. I hadn't thought about whether they are nerds or not but... they probably are. 
Neem is blind but wears glasses to protect his eyes from debris and bright lights.

Jasper wears reading glasses and is a librarian. He needs them, but not all the time. Maybe he just knows they make him look studious. Or maybe it's just a pain to constantly take them off only to put them on again in five minutes.