on cb750s, 550s, basically any sohc 4's the exhaust runs right next to the oil filter. Sometimes custom pipes will run too close to the oil filter and raise the oil temp. This is the only case in which I think header wrap will make a significant difference in oil temp. On a standard cb though it does seem to have a marginal effect. However an oil cooler kit is readily available for these bikes and looks 10 times more race than header wrap (although the copper header wrap does look cool).
my advice, find out what is making your bike "overheat" in the frist place and fix that instead. Most often it is some sort of lean condition (like air sucking past the intake manifolds). how do you know when a SOHC4 is overheating? It detonates more then me after a baked bean eating contest. Seriously, it will sound like you are riding a giant coffee can with a bunch of rocks in it.