If you want to take college senior pictures, go for it! This is not a matter of etiquette, it is a matter of what you would like to do. Keep in mind that photo sessions and prints cost money, and so you should consider this when deciding whether or not you want to get pictures done. No one is going to think you're rude for not sending them a picture with your graduation announcement. However, your grandma that you haven't seen for 4 years because you've been away at college may appreciate one.