Fat = a yellowing euro mount over time. The less the fat, the less the yellowing.
The first one i did about 10 years ago with the slow boil removes too many of the thin sinus bones (to me).
Some folks go to a taxidermist for beetles to do the tissue removal.
Another option vs boiling or beetles is maceration...let bacteria remove all of the tissue. That also seemed to help a bit with reducing fat in the skull. Maceration is both quite nasty smelling and effective.
After soaking at least 3-4 weeks of a warm dawn/water solution, that will pull most of the fat.
It is a bit of a pain and cost, however soaking in acetone at the end before whitening will even pull out more fat.
There is some museum-quality sealant available soluble in acetone that will seal the skull very well...it is Acryloid B-72. Most taxidermists won't go to that length for a euro mount.