Ryan, I look for the lightest I can find, but try to look out for poor butcher jobs (shiners) and real fatty ones. Unfortunately, you can't tell much about the 3 rack cryovac packs though, but that's what I usually end up with because of the price.
I've also found that it's smart to check out the different stores running sales around the typical warmer holidays. That's when we find more of the lighter racks in the stores. Remember that ribs are better fresh than frozen, though, so I'd resist the temptation to "stock up".
I plan on doing a bit of checking around this year looking for a meat market that can find me true "3 and a half downs" at a decent price, but I usually end up with packs averaging 4 lbs. or so a slab if I do some looking. If you end up with 5 lb. slabs, you better plan on using some foil or they'll be pretty dry by the time they're tender.