I usually don't get the extended warranties myself. I usually check stuff out pretty well during the first 30 days or so, and if there is anything wrong with something that isn't easily fixed I get it taken care of then.
After that, I figure there isn't much that can go wrong that I can't fix for the $80-130 the warranty costs. I can even replace the motor if it goes out and still be ahead.
Just food for thought... the machines are pretty modular so unless the bed just up and decides to crack on it's own I think you could fix just about anything for less than the warranty.
Plus, if you've modified it... you only have to re-mod whatever is broken and not the whole machine.
I have an 8x12, though... I'm not sure that it's the same with the 9x20.