As much as Kelly's advice is sound, it all boils down to substrate. If your substrate is solid, consistent and compatable to your top coat...ie fully cured single stage urethane, prepped with proper grit of sand paper (600 wet, 400 dry for base coat) sealer is not required.
Look at it this way, if a body shop is doing a spot repair on a fender and have primed the repaired damaged area with a quality 2K primer, do they seal the fender? NO...the sealer would cover the color in the blend area not allowing for a blend of color. Sealer is meant to give the vehicle being painted a uniform substrate, if you have a uniform substrate....why seal it? It's just another product that costs money, keeps you in the paint booth longer and gives you more opportunity to get dirt or whatever other imperfections that might occur in your new paint job.
Hope this helps.