S! Tiki and S! Elise

i mean it helps. but it aint the end of the world if you dont have them.

I think Sword Valor (Gray/E. Chrom), Lance Valor (S. Robin) and Axe Valor (S Tiki) are the most important. It helped me a lot.

I needed 1400 SP to give some new skills to my Dorcas +Res with Warding Breath and Wind Blessing. With all the bonus, I got 1400 SP in three or four battles on the Special Training in about 5 minutes. It was CRAZY!

Depends on what you have.
If you have lots of axe & tome user, they'll help a lot.