Friends with your Ex

Is it cool to still be friends with your ex girlfriend or ex boyfriend? To me i do not know what to think since i am a friendly with one ex girlfriend but the other one, not so much. I am only writing about this because i just met up with my ex girlfriend of a year because she needed to get a t shirt back. Even though we broke up last february she finally asked for it back. Now we are not still friends, and i am not sure if i would want to stay friends with her, is it because she broke up with me and maybe i might not be over it completely because i can miss her at times? but then again i am fine on my own and am physically and mentally stable. So then there is one ex girlfriend i am still friends with. We dated for 3 years and we are friends but a girl that i dated for one we barely ever talk. Now are we friends because i broke up with her and thats why i am totally fine with it? even though we had our rough patches of fighting and her hating me, we still are friendly and get along. I think its because outside of our relationship we also had this great friendship and bond with each other and thats what allows us to still talk.. who knows these days..... to be friends or not to be friends.