Coming from a 50NL 6-max perspective.

I hear a lot that I should refrain from c-betting draw heavy flops, but what exactly constitutes a draw heavy flop? I mean technically every flop has some various back door draws, but at what point should I be thinking "Ooh, draw heavy, put the brakes on!". Here's a few examples that I'm not sure about; hopefully somebody with some experience can comment so I can get the idea:

QdJd7s
Ad4s6c
8d8c7h
AhKh7s
5h6h5s

Any rules of thumb?