Primer Questions

I would use sealer especially if:
>you had any bare metal showing(usually edges, body lines, etc) showing
>you sand though to a different colored primer
>you sand into the filler
>you have both primered areas and painted areas that are different colors

Mostly it seals in the underlying materials from the paint you are putting on, and provides a uniform color to give the paint a base.

Sounds like you have done it right and really don't need to seal it. Additionally, another coat of anything is more chance of dirt, bugs, drips, runs, and the whole ugly mess that you may have to deal with while it is drying.