I vary knots based on several things:

1) Cut of the shirt collar, points get Four-In-Hand or 1/2 Windsor, spread collars get 1/2 or full Windsor.

2) The cut of the suit itself - a long, thin lapel doesn't usually want a fat knot.

3) The tie itself. I've got thin ties that go with more Modern suits (60s) that I'd never tie in a Full Windsor, likewise I've got some very lush, thick silk ties that would look ridiculous with a Four-In-Hand knot.