Hi csiman, welcome to the gyo forum. I've never seen a greenhouse on decking before but as long as the decking is solid and does not move as you walk on it and as long as it is level you will be OK. Things I would consider is keeping it clean. The inside of a greenhouse is a spawning ground for all sorts of bugs. Also, the joint between the greenhouse base and the decking would be a moisture trap that could lead to the decking rotting prematurely at that point.
There has been a thread on Billyoh greenhouse recently on here you might want to subscribe to:
Billyoh Greenhouse