So I've been thinking a little on when best to promote and reclass, and... I think I might have a good rule of thumb. In that there isn't one. It's true that staying in the base classes before promoting is most likely the optimal way EXP wise, but that really only matters when you get past the story, and into DLC, doesn't it?
There is another concern:. Keeping characters viable as they level up. And a promoted skill or two is a nice thing to have there. Like how I said before that Vaike almost got himself killed by overkilling on the Enemy Phase. I classed him up to Hero to go for Sol, so he could heal himself if that happens again, even though I kind of want him to end as a Berserker.
I'm thinking the same with Chrom. Aether is an amazing skill. Might go fishing in his Calavier tree for Aegis and Dual Guard+, but for being the required character in the story missions, he needs to be good early, then min/max later, despite the EXP disadvantage of having to go back to Great Lord. But I suppose a level 20/1 Great Lord with high stats and good skills is just as good as a 20/20 one with the same. It's pretty much an aesthetic choice in the end there... At least he's not taking the same massive detour as his little sister. She'll end up Sage after trying nearly every other promoted class. Cleric -> War Cleric -> Valkyrie -> Dark Flier -> Sage. That is one heck of a trip through classes.
Basically, it's all a judgement call. Sacrfice current usefulness for faster future power.
Got the same issue with Nowi and Panne. Their base classes are awesome, but they need to go fishing for skills elsewhere. Do I toss Nowi into Wyvern Rider ASAP or do I use her awesome dragon form for easier kills? Her level 15 skill isn't useful outside of Manakete, and even there it's... situational, but if I get it first, I won't have to go hunting for it later, I can just class her back to Manakete and be done. Same with Panne, except her level 15 skill is pretty useful. No idea where to go from there. At least Donnel's Villager class is bad enough to warrant a Second Seal as soon as he gets Underdog. A pity there's no other on foot lance class. Unless I missed one.