For a base class, you can reclass out using second/master whenever you want to. The difference is that you gain more experience in your base class than you would in your promoted class, meaning it's easier to gain stats.
For a promoted class, I would reclass after getting to level 15, as that is when you learn that class's specific ability. You can reclass at 20 if you feel like it, it won't make much of a difference.
Is it still best to wait until a character's class is maxed out before using a master seal? How about second seals. Basically, I just got my first ones and I want to know if I should use them or wait.
In any other fire emblem game, yes. In awakening, because of second seals, you don't need to worry much about growths. Just reclass/promote when you get both class skills, and keep doing that until you have all the skills you want. Then reclass to whatever you like best. Whether or not you should use them depends; for example, on, say, Donnel, you want him out of villager ASAP because the class all around sucks, and with Lissa you want to reclass to pegasus knight so that you can get to dark flier and grab galeforce. On the other hand, with chrom, you probably want to go straight to great lord for Aether and Rightful King and go to great knight then paladin, or vice-versa depending on where you want to end.