I go for the best setting for consistency rather than the best setting for range, because if you know where the shots are going to go, you can adjust the range by moving the aim point up or down. On my M4s, I only use open sights and in fact I often fold the the front sight down and line the rear sight up with the U shape left by that gap where the front sight would be. That's not going to be everyone's preference of course, but it seems to work for me, because I like to push up. And I guess that's the key really; set it up so it works best for you.
Strictly speaking of course, assault rifles (or at least the first ones such as the STG and the AK) were never intended to be for pinpoint accuracy, they were about keeping people's heads down so that you could move a squad up, pin the enemy down and get into grenade tossing range, hence the name assault rifle. So you should regard an assault rifle as one part of your equipment, and grenades as its partner, because that is how an assault rifle is meant to be used.
A sniper rifle or a DMR is a different story of course, but the important point with those is the consistency of shot too, because unlike with a real sniper, it's unlikely that you will have a spotter making windage and ballistic drop correction calculations for you in a fight, you need to know where that round is going to go all by yourself. So, what you need to be aware of, is the typical range you are likely to engage people at, at the site you go to regularly. So, pace that distance out, then go to somewhere where you can set up a target at that same range, this may in fact be done at the airsoft site itself, since you can always get there early, and do this whilst awaiting a morning brief or, when the lunch break is on. What is helpful, is to have someone with a PMR radio alongside your target whilst you zero the rifle in, calling corrections, because that saves you tramping back and forth to check where the hits are going. If you don't have radios, then work out a signalling system, such as pointing in the direction the shot is off in, and of course one for cease fire, or you will not be popular lol.
Take something like a piece of cardboard and mark rings on it with a felt tipped pen, and be sure to also take something to secure it properly, because even in a light breeze it will be blowing around if not secured properly. Make sure the scope is fitted securely too, otherwise if it moves on the mount, it will all be a wasted effort, so take a notepad and write down any settings you adjust on the sight or any of the mount screws, you can mark the adjustment dials with tippex if you like, which does help. If you are sniping, sort your ammo out manually as well, wash it too, since a lot of ammo is covered in stuff such as mold release compound (which is a bit like soapy detergent), and that will affect consistency of shots if some BBs have that on them and others don't.
Other than that, it's just a tedious: shot, correction, shot, correction, until it's zeroed okay.