The fast and easy rule is that if what your loading for ISN'T a bolt action you need to FL resize. It'll save you a lot of headaces.
I've never heard of this rule...and have been reloading for many many years...I've only heard of having to FL resize everything if your using a autoloader..or if the ammunition will be used in another rifle...Not in a single shot rifle.
How one reloads should be determined by the individual rifle...and that individuals preferences and requirements.
I neck size all of my brass for all of my Handi's...and set the dies up to chamber the round without any heaspace.I've done this for a long time...and my accuracy is excellent...This is what works best for me...If the chamber and bore have a concentric bore/chamber axis..then you can do well neck sizing...if either the chamber or bore are out of alignment..then you may have to full length resize to get good accuracy...Arbitrarily saying one is best isn't really true..It's what works for that rifle...
Try both ways...but do it correctly...and see what works good for you...
Mac