The 5/10 rule with medium PP

Hello
I have a common scenario:
I call a raise preflop with say 7 7, according to the 5/10 rule. Then the flop comes 2 5 9, the preflopraiser makes his ½ pot C-bet. Now I fold the best hand to often. If I hit some kind of straight draw I might play back expecting the leverage from the straight cards to give me FE from at least AK/AQ. What are your thoughts about when to play back against the C-bet?
What reads do u look for?
What kind of flops do u play back at?

//Johan