I guess it likely wouldn't hurt if you were doing so on your own accord though if you can reach them, you could still try and see if they could send you their OK yet. Just for good manners. Proper crediting is definitely always the correct step to take.

Unless they said something to you about it, [and this is my opinion, take it as you see it] You commissioned it, therefor you have the rights to post it.

[...things get dicey when you wanna use it in a non personal way, ie, putting on a coffee mug and selling it]

i have also been wondering a lot about this cause i have a lot of old commissions stockpiled, but i agree with the other comments
most of the time i try to ask first, but i think unless they explicitly have a "dont repost this elsewhere" clause or ask you about it later, it should be fine to post here!! and id love to see more art you got cause your characters are cool and cute >:3