I've done it both ways; on my first build I did a seal coat, on my second I didn't.  I did not notice a huge difference either way in adhesion or strength.  I think if you have conditions that could result in bubble formation or uneven resin absorption (working during warming temperatures, a strip-built boat, or, as I found, bubinga veneer) it may be worth the time to do a seal coat so you can sand out any problem areas and/or get even absorption before you apply the glass.  If you choose to go the seal coat way, make sure it's as thin as possible; use a plastic squeegee (the kind used to smooth bondo - very cheap at the auto parts store) to take off all the excess resin.  Sand lightly before applying glass to get good adhesion or put glass on just as seal coat loses its tack for chemical bonding to take place if there are no bubbles you need to sand out. 

I've found a good reason to do a seal coat first (actually even before stitching) - if you work outdoors, the occasional rain that catches your boat unawares will not stain the wood. 

Mihai