Is the traditional form of 复 always used in “翻天復地" and why? As asked, 

Is the traditional form of 复 always used in “翻天復地" and why?

I assume it’s because the simplified form is one of those odd ones that isn’t a one to one mapping, so the traditional form is used for clarity?

If so, is there a rule about when the traditional character is preferred?
@johan is correct.  翻 and 覆 are the same in traditional and simplified.  

What I was seeing is 飜, which is a variant that looks more complicated, so I assumed it was the traditional.

And  I was also looking at the wrong entry for 覆,   here, which shows that 覆 is only used as a variant of 复 or 復 when it means  “to reply to a letter” and not as used in the idiom. 

Just a noob making noob mistakes, which I apologize for.  Thanks for taking the time to answer my question :)