Do I need to take special steps to fully restore an iPhone after jailbreaking it?
This is the first I hear about this so-called "counter". I don't believe in such and I'm pretty sure when you restore an iPhone using DFU modus, the iPhone is completely wiped clean.   

As a matter of facts, I've jailbroken all my iPhones so far and passed by the Apple Genius bar several times. Once I even forgot to restore the device when asking a Genius for help. And he just smiled at me and said "jailbreak, right? What's your favorite app?" :)

So, no. I don't think there is such a counter and you're safe with a DFU Full restore.