A very simple tip to help avoid distortion is to always release your fabric from the frame every time you finish stitching, this is ultimately what made the most difference to the end shape of my fabric when I was using a tambour.
Well, further to my previous post about the Last Judgement (which I've just remembered I never posted a picture of) - I've given up on the big frame! We've just moved from our winter sitting room into the summer conservatory, where the sofa is completely different and quite a bit smaller. As a result there isn't enough room for a very unwieldy 36" frame, so I've got myself an 11 by 17" Q-snap frame, and rolled up the rest of the fabric and secured it with bulldog clips (no idea what they're called in the US). So far it seems to be working pretty well, and at least it keeps me stitching. I'll have to keep the big frame for winter projects.
I should add, I don't grid, but I do mark the page outlines with a 10 under, 10 over thread and use that to count from. I found I was making too many counting mistakes not to do something, but I'm too lazy to do a proper grid. This seems to keep mistakes to a minimum.