We have just decorated our front room, which is where the stairs are too. My DS is 2.3 and has always been very good with the stairs. We have left the bottom gate off since decorating (couple of weeks ago) and he goes up occasionally (top gate is still there and shut) and never has a problem.
I sometimes nip up to the loo, or nip out the back to put the washing out which leaves DS on his own in the frontroom. Basically, would you put the gate back on as he is still quite young or just leave it off now?