Even though everyone keeps asking this, and even though everyone keeps saying, "Level 10 is fine because you aren't limited to a certain amount of level ups"... well, how many people are actually going to grind that much anyway?
If you play it like a regular FE, just going through the story, it's still best to wait to 20 to promote, and when grinding, I will probably go to 20 anyway unless most of the stats are capped.
The key thing you've left out is just how good many of the second-tier skills are; some are close to game-breaking. Acquiring those skills quicker is much more valuable than having 5-10 marginally faster level-ups. --- West of the Brick A farewell to Lin: http://i.imgur.com/JiawS.jpg