For a Guitar Hero game on the Xbox 360, do I require a Dongle like the PS2/PS3 to use a Guitar Hero Wireless Guitar?
No. The Guitar Hero controllers for Xbox 360 are wireless, naturally. They will connect to the console in the same fashion that a wireless Xbox 360 controller will.
That said, from what I have seen, PlayStation 3 Guitar Hero controllers operate in the same way; they also do not need a dongle.