I haven't read it yet myself, but my friend wrote this review that seems to indicate it's pretty important: 
 
http://www.tradereadingorder.com/blog/review-secret-six-six-degrees-of-devastation/ 
 
still, according to the reading order ( http://www.tradereadingorder.com/character/secret-six/  ) 
 
Unhinged is the start of the ongoing and those are usually pretty good jumping on points. I'm sure it will spoil events of six degrees, though, so personally I wouldn't be able to read them out of order. Then again, I'm pretty much obsessed with reading things in order.

Nope. Already answered. But you do not. I started with Unhinged and was just fine. 

Unhinged would be just fine because it is the start of the new series. But that said, there are somethings that you would not completely understand like Cheshire's relation to Catman or why Scandal is depressed at the start of Unhinged.

For me it was important to read them in order, of course the world won't end or anything if you don't.  But why would you skip them, you can still pick up nearly any book you want if you search for it.  

Six degrees of Devastation is a fun story and makes Unhinged a bit more fun, but it;s not by any means necessary.

just read in the following order Villains United, Then Six degrees of devastation, then the whole series run