Thanks to you both. I'm guessing I can't exactly break my playthrough either way. I don't even have access to obtaining seals yet so I'm not sure how someone would even be able to promote early on.
Oh, by the way, in case you missed it a handful of posts up, this seems relevant to your interests:
.....yeah, never Second Seal from a promoted class into an unpromoted class
experience formula (and others) can be found here: https://serenesforest.net/awakening/miscellaneous/calculations/
tl;dr every time you Second Seal you get half your level at the time added to your internal level