There seems to be a lot of misinformation on the internet about when is best to use a Master Seal in order to maximize the stats of my units. Does anyone have any concrete information?
In most Fire Emblems, if you were wanting to maximize your stats, you'd only use a Master Seal once the character has reached max level in their current class. This is because it was a one-way street. You had to get as many levels out of them as possible, and using a Master Seal at any point when able had the exact same effect: Level 1 of the new class.
Awakening is different; you can sideclass your units, and even restart the same class once you hit max level. With enough Seals (Second or Master), every unit can hit max stats. This means that you shouldn't be focusing on maximizing their stats, as it's pretty easy to do so. What you should be doing is focus on what skills you want to give the unit. Once they've gained all the skills from a specific class that you want, it's time to move on and choose a new class.
Once you have all the skills you want for the unit, upgrade them into their final class, and leave them there. If they haven't hit max stats at max level, use a second seal, choose the same class, and they'll start at level 1 again, with all their stats intact.