Boiling makes the bones brittle. Plus it stinks, makes a mess, and is a pain to get all the gunk out.
Degreasing takes a while if you don't boil. All depends on the amount of grease in the bone, could be days, weeks, even a month or more. Just depends but if you don't want the skull turning yellow, you better do it.
As far as the best method, it doesn't stink up my house to do it with an aquarium heater in a bucket in my basement with ammonia and dawn. That keeps my wife happy, which in turn, makes it the best method FOR ME! haha
"Knowing when to shoot and when to wait is one sign of maturity in a bowhunter. Be patient and pick your aim point carefully." ~ Randy Ulmer
"I suppose it's the way of hunters. We are very odd fellows." ~ Peter Hathaway Capstick