I generally tend to nod.... Unless they (like most of the chav cyclists around here) are on silly little BMX's with no breaks, lights or other gear or squeaky old MTB's. Usually these riders are ridling all over the cycletrack or even on the wrong side (on the right). They usually get a cautious froun instead. 
I will always nod back though, even if it is one of these chavs/ninjas.

I will always nod back though, even if it is one of these chavs/ninjas.