I built a Shearwater 14 last winter and yes, the deck is under quite a bit of tenstion with the deck forms in place and it does spread when you take them out.  However, I found that it wasn't too hard to get back to shape when I got to the stage of re-attaching the deck and hull.  It did take some work, lots of clear packing tape, and a thin putty knife to slip in and pry in the appropriate direction (slip the putty knife in the shear seam, then push up on the handle so the hull out is pried out and the deck pushed in).  I started with packing tape at the bow and stern and worked my way to the middle from both ends at once, taping every other stitch location and then went back and filled in with packing tape at the missed stitch locations.  This seemed to hold it well enough to get the inside seam taped with the fiberglass tape. 

Note that I applied a saturation coat of epoxy to the top of the hull before I applied the packing tape because I found that taping on the bare wood tended to pull up a few flecks of the wood, but this is not necessary, I don't think the tape will pull up enough wood to be noticeable; just my little concern as a first time builder.  There are photos of my build here if you're interested:  http://www.westcoastpaddler.com/community/viewtopic.php?f=7&t=3517

Thank you KathyD-now I won't try anything silly, like steaming the deck plywood. By the way, I like using the "shrink tape" mentioned in the manual. Its easy to get at HD (the stuff they use to hold big rolls of carpet together). I used it to help hold the hull together while the "tack welds' cured, and to hold down mt marquetry inlay.