for the ar15, the important things are:
full length resizing with a good quality sizing die. in order to ensure reliability with feeding, you want the cases sized completely. it's helpful to know what chamber is in your particular rifle too. i bump the shoulders on my .223 cases back ~ .002" to ensure proper headspacing.
trimming! if you're using a dillon to load your ammo, you typically set the powder funnel so it's just barely clearing the cases when the handle is all the way down. if you set that height based on a case thats 1.74" (minimum length), then run a case thats, say, 1.78" (.002 over max), you can actually squish the case neck out a bit and ruin the neck tension you set with your sizing die. you dont want to leave the powder funnel high enough to let any case through, because powder can hang up between the funnel and the case, and when you bring the ram down, you will spill powder onto the shellplate. it is only a tiny bit at a time, but it adds up fast when you're loading a few hundred rounds, and that powder can gum things up.
loading for maximum accuracy? or just plinking ammo. for short range (under 300yds), loading them to mag length will be fine. for long range (5-600yds), you're better off with a heavier (IE longer) bullet, and seating depth becomes a big factor. I seat my long range ammo ~ .010" off the lands. the best (reliable) way to check that is with a bullet comparator (Hornady makes a nice one for ~ $45 or so for the whole setup that will accurately measure the length of the chamber to the rifling)
BLC-2 is a ball powder if i recall correctly, so repeatability through most any powder measure out there should be okay.