Okay, so from what I read, Promoting at Level 15 [after receiving your Second Skill in that Class] instead of waiting until Level 20 doesn't impact the Cumulative Level in any different way, since 20+ is always added to it [thanks to the more powerful unit your gain].
However, for Base Classes [and Second Class Seals], it's supposedly best to use the Seal early on, since Cumulative Levels do carry over. So the more you level in one class after learning both Skills, the more XP it will take to reach the remaining Level 1/10 Skills in the remaining Base Classes.
For instance:
Level 15 Tactician [after learning Skill #2] - 20 XP per enemy
Level 20 Tactician - 15 XP per enemy
Grandmaster [after Promotion] - 10 XP per enemy
If I used a Master Seal at 15, as a Grandmaster, it might lower the XP gain to 10 per enemy [since the Cumulative Level would increase to 20+ immediately]. The would also also apply if I used it on the Level 20 Tactician. So, in essence, if I were to stay as one Class throughout the entire game [in this case, Grandmaster], it might be better to wait until 20 to Promote, since you would easily gain the stats you would receive from Levels 15-20, and at the lower XP cost.
However, if I'm using a Second Class Seal and Re-Class as a Mercenary:
Level 10 Tactician - 30 XP per enemy
Level 20 Tactician - 15 XP per enemy
If I were to Re-Class at Level 10, I would be gaining XP at the new Base Class [Mercenary Level 1] as a Level 11 Tactician [since the game remembers my previous level]. This means I would learn the new skills a lot faster, since less XP = more levels. On the other hand, if I were to wait until Level 20 and Re-Class then, I would essentially be earning the XP for the Base Class [Mercenary Level 1] as a Level 21 Tactician. This would mean more XP to gain each Mercenary Skill, and thus, a longer time.
So from what I see, if all you're looking for are Skills, feel free to Class Change as soon as you reach Level 10 and earn your second Skill unlock [or Level 15 as Donnel]. However, if you're interested in Promotion, wait until you reach Maximum Level before doing so, so you can earn/gain XP at the much lower Base Class XP level.
What I'd like to know is what happens when you Promote and THEN Re-Class into a Base Class. Do you keep the lower XP gain from the Promotion, or does it go back to the Base Class values?
...My head hurts.