IMO, it is pretty important, although with light kicking rifles, a bit short is ok.
LOP is the measurement from trigger to center of the rear face of the butt pad. It impacts head to scope fit, as well as trigger pulling arm comfort and your ability to mount the rifle, especially offhand. I find it mostly impacts speed of building a position, but probably impacts field accuracy.
I find the best way to find lop is to mount a rifle over and over while adding and removing spacers…cardboard or something about 0.25" thick. If you have a scope, it may have to be moved some. Once you find that ideal lop, then ask yourself, will I likely wear a coat, body armor, etc? If so, subtract some. Will I likely be pulling in firmly, add about 1/8" to 1/4". Are my shoulder positions right? Long lop's make you less square to target, but too short can be hard to be repeatably that square to target. Find your natural point of aim.
Good luck. At 6'3", my hunting rifles are about 14".