It depends.
I've got 4 .223 semi-autos, and I want the ammo to interchange and feed reliably, so I only use SB dies.
If I had 1 .223 I was reloading for, I could get away with standard FL dies, properly adjusted.
Here is RCBS answer:
Quote:
The Small Base Die set is intended for use for ammunition to be used in auto, semi-auto, and lever action rifles so that the loaded round chambers and extracts easily. The Small Base Sizer Die sizes the case from the shoulder to the head of the case a couple of thousandths smaller than a Full Length Sizer Die. In certain calibers it also sets the shoulder of the case back a thousandth or two more than the Full Length Sizer Die.