I think it depends on what you're trying to accompish with the design. I use a version of the Firestarter (3025) that swaps out the flamers for inferno packs. SRM2s are perfect for that as they give you multiple chances to hit and a wide arc to target (with one in each arm and one more in the CT). On the other hand, with the Javelin (3025), I go for raw one-pop firepower. Hop in, blast for crits, hop back out again to cool down. SRM6s make a lot more sense. A couple of good roles, or some help from buds, and you might force a PSR, which can ruin the target mechs day real fast.
With LRMs, I also tailor the size of the launcher to the profile of the mech. LRM20s on Archers make perfect sense. When you hit, you want to get maximum yield. Again, a lucky shot or two can force a PSR, killing your opponant's movement mod for the next round and making him vulnerable to flankers. Also, LRM20s have a much better chance of wearing the armor of a particular location down (using 5pt groups instead of the probable 3 pt groups of the LRM5). That lets you get in a postion to start generating crits. On the other hand, long range sniping or critseeker mechs may benefit from the LRM5 spread, letting them get multiple chances to hit and cause crits. This is particularly effective against vehicles. Say, a Warrior with LRM5 packs would be much more likely to get a mission kill than one armed solely with an LRM15.