Yes, Promoting early is usually always a bad idea. If you're planning on Promoting and are still in a Base Class, wait until Level 20 and then Promote.
The reason for this is because when you Promote a Unit, it's automatically labeled as a 20+ Unit. If you Promote at 15 or 20, it doesn't matter. They will immediately be seen as Level 21. That means you essentially lose out on the easier level gains from 15-20 if you were to promote earlier, since the game now sees your character as being 20+.
If you're interested in gathering Skills for your Unit, it might be better to re-class to whichever Base Classes you'd like first [before Promotion], gain the Level 1/10 Skills you need, and then, once you're happy, level to 20 in your Class of choice and Promote. You could then stay in the Promotion Class area indefinitely and use Second Class Seals [after Level 10 in your Promoted Class] to switch between other Promoted Classes as you deem necessary to gain even more Skills. This would ensure your leveling in the Base Classes won't be slowed down by "Promotion XP Fatigue", and since XP is usually automatically reduced when you Promote, switching between other Promoted Classes shouldn't impact you too much at all.
I hope I explained that well. I did so to the best of my ability.
