If you're going to use MMLs, and don't want Artemis on them, there are very few cases when you should use anything but MML3s.
Consider:
5xMML3: 7.5 tons, 10 crits, 10 heat, 20/10 damage
3xMML5: 9 tons, 9 crits, 9 heat, 19/9.5 damage
2xMML7: 9 tons, 8 crits, 8 heat, 17/8.5 damage
You save 1.5 tons and get more damage. Sure you might need an extra DHS, but unless you're outfitting an omni you should be able to change something else to get those spaces.