If one of the exes is still carrying a torch for the other, then it is not OK. If one of the exes is trying to pull some shady stuff and undermine the current relationship, not OK.
Otherwise, why not.
To Trenell, MizKerri and geeky:
I pray none of you ever has to live in a communist state.
Geeky is my hero. She's the true badass. The badass who doesn't even need to be a badass. There aren't enough O's in cool to describe her.