If bottle was used to feed baby formula, does it still need to be sterilised? If yes, why?

For me personally, I stopped sterilizing his bottle when he started on solids. Reason is because my son has already started stuffing everything in sight into his mouth, and it's also impossible for me to sterilize his food all the time. I didn't want to too "protective" against germs so that he can build on his immunity better.