Considering that most other units will be deploying a minimum of 5 wide, why would you stop at 6 wide? Why not go 7 or more? (An honest question.)
I wouldn't consider more than 6-wide for anything but Flagellants because, honestly, nothing else in our Infantry can actually dish out enough damage to pay for the -1CR for not having the 3rd rank (because I really don't think I'd invest in 21 Greatswords, but 18-20 sounds great and a safe bet the third rank is still there by the time combat is joined). Also, as I've learned to my annoyance today, 7-wide is a pain to maneuver during a charge, and if you hit a block anywhere but from the front, you're losing out on attacks. 7-wide will also result in your troops licking out one model against enemy 5-wide formations, and might even block the movement of your other 7-wide units who are trying to charge a flank, for example.
Flagellants have no need for CR, and thus can go as wide as they like.
Also, good point about the Free Company, Eight. I completely forgot about them.