Best character for me was probably Titania or Ike, both were near invincible.
Mage: Sorren
Swordsmen: Mia (Although, I like to use Edward too, and usually use both)
Sniper: Shinon (don't have to train him like you do Rolf)
Priest: Mist
Pegasus Knight: Marcia (get to use her the most)
General: Gatrie
Laguz: Don't use them, since I don't like manage their transformation gauges.
I typically support the following
Ike & Sorren
Shinon & Gatrie
Titania & Oscar
Other than that, I really just randomly pick other people who I think I'll be keeping close together.
As for your question. Yes, I use Master Crown/Seals whenever I have a unit at level 20. It saves you a few experience points, and I don't need the cash (If you can sell them, I can never remember)
I don't use Spectre Cards either, but that is because I forget about them. In fact, I pretty much don't use any of the power up items either, because I don't want to use them until I absolutely have to, which I typically don't.