I have an iphone 5 & an ipad ... I'm currently out of the country and have suspended my USA cellular service. I received an imessage yesterday from a friend while I was on local public wifi ...
Q ... am I being charged for imessages on my suspended cell service or are all imessages free no matter where I am?
If you were on WiFi, then you will not be charged by your cell phone provider.
Note, however, that iMessage is not "free no matter what". It always uses data. That may mean that it comes out of your data allowance on cellular, comes out of your home ISP if on your home WiFi, or uses a third party WiFi hotspot and comes out of their data allowance with their ISP.
But no, you will not be charged by your cellular provider for data you used on a public WiFi.