No.
Levels before using a Master Seal don't matter; you just get set to a class level of 21. When using a Second Seal, however, the game takes your current class level, subtracts 1, and cuts it in half, then adds it to your internal level.
So a LV16 Hero who's never reclassed will be counted as LV36 no matter what their level was before promoting, and if they reclass, they'll be counted as 18 levels higher than their new class level. So if they become a LV1 Cavalier, they'll count as LV19; if they become a LV1 Paladin, they'll count as LV39.
Meanwhile, these internal levels stop counting at a certain point based on difficulty level: 20, 30, or 50 for Normal, Hard, and Lunatic, respectively. So if you've reclassed a character a bunch of times on Hard, then when they end up back as a LV1 Hero, they'll count as LV51; if you do the same on Lunatic, they'll count as LV71 as a LV1 Hero.