Only reason to go 5-1-1 is the undaunted passive which is pretty important. Just look at the last passive, as far as leveling them up its for the small scale passives you get with wearing X amount if said arnor.

It's all about just maximizing potential and skill point allocation.

All the armor helps, so chest-heavy and legs-medium is good.

While leveling you should use 7 Medium or 7 Light. Even with the undaunted passive your stats are so extremely low that 4% extra to them really isn't ... anything.

Since I know light armor off the top of my head, that is 6% spell cost reduction you would not have. Read the passives as they're extremely good. Quite frankly that's so good and "build defining" that most people realize being able to use an extra spell damage enchant on their jewelry vs a spell cost reduction one is much better than the 2% stats a medium piece provides (heavy gives health so most still use that regardless 6-1-0) until CP's can cover the slack. By the time CP's cover the slack (300+) you're obviously not leveling anymore or you level at a glacier's pace.

There's also the matter of skill points you do not have while leveling to invest in the EXTREMELY minor benefits of wearing 1 piece of heavy and 1 of medium/light.

Edit: Misunderstood the question. To level up Heavy/Medium armor what you should do is craft a set of gear with the training trait and upgrade it to blue and carry it around with you. When it comes time to turn in a quest you simply equip that armor for the quest experience. Wykkid outfitter allows you to save gear sets so this becomes an extremely simple and quick process. You can also do this to level up skills in the same way, wykkid will save a skill bar for this purpose as well.

Equipping just one piece of each heavy/medium all the time for the purpose of leveling the skill line is honestly a bad idea, even without the addon.