First I'll say that 70-100 pounds of gear is a very heavy load. Without resorting to a lot of really high dollar UL backpacking stuff, I went coast to coast camping and cooking with 14 pounds of gear and was pretty comfortable, so I really don't see how anyone needs 70-100 pounds of gear for touring in developed countries. While I don't recommend taking that much stuff, if you do then I'd definitely try to put the heavy compact items in the front and low as possible and big light stuff in the back. I wouldn't get hung up on the percentages though.

For reasonable loads (25-40#) it is way less of a big deal and for light loads (<25#) probably pretty much a non issue. For ultralight camping it is definitely a non issue.

Over the course of my touring evolving from heavy to ultralight I have used 4 panniers, two rear panniers, two front panniers with the tent on the back rack, no panniers with stuffsack(s) on back rack and bar roll.