"Every single person that knows FE games, never promotes before 20 because you loss possible stat gains, so the only person then that would promote before 20 is someone who doesn't know the game very well."
I'm currently about to finish my 3rd FE game (all on the GBA) and I've never waited until level 20 to promote, and have never had difficutly beating the game. Just have the strongest character or two take on the bosses and you'll be fine. It's never been my goal to max out a character's stats, I just want to get through the game. Besides, some characters may be too slow to gain levels if you don't promote them b/c before promotion they don't have enough (or any) chances to attack (such as a weak swordsman who can later attack freely with a bow after a promotion, or a healer who can attack with magic after promotion - and yes I do have the healers heal every little injury, even 1 point of damage).