[Mathieu]

* grunch *

I've never played six max so I could be way off...

Here I would check with the intention of raising. CO will call your check raise with under pairs, even more so if Button folds since he will worry that you are semi-bluffing the flush draw.

Also, on a K high flop CO might bet some Ax hands that he would fold for 1 bet.

I doubt that this gets checked through, but if it does it's not so bad since few cards can hurt you.