well, my opinion is yes, but honesty is the best policy. if you having a partner, must let he/she knows what exactly happening between you and your best friend. so your SO would not easily get jealous and have the feeling of insecure.

and, for ex-lovers. i'm not so sure leh. initially i thought i wouldn't befriend back with my ex. but in these 5 months, he texts me daily, we chat alot everyday as in everything. gradually, i tend to forget he's my ex lover and i hope we can maintain our friendship now cause we're the one who understand each other the most.