when i use the longarm, i start at the top. by the time i get to the bottom, it HAS eaten almost all the extra in that direction. also, i 'float' the top, so sometimes it creeps to one side or another. having that extra gives me some margin for error. i usually start with 4-5" extra on each side for my quilts. since i shoot for 96" to 100" square, i use a king-size batt. i wait for a 50% sale at joann for the batting before i sandwich the quilt. sometimes i piece a queen sized one if i have the right leftovers.
when i work on a regular machine, i start in the center and work around the center, outward. that way i need much less extra batting, maybe only 1-1/2" each side. i pin a lot first, though.