When I first got my SIB, about 8 years ago (crikey how time flies), I stored it deflated. I used to transport it in the car (Mondeo Estate) and then I'd have to inflate it, use it, deflate it and take it home. Then I started to store it inflated so that it didn't keep getting folded and creased. This meant that I'd have to deflate it, pack it into the car, inflate it to use it, deflate it to get it home then inflate it again. Despite having an electric pump (but I always used the manual styrrup pump for the last few 'blows') it was a real pain in the butt.
So I purchased a De Graaff SIB trailer and now I keep it inflated on the trailer. This means MUCH less fraffing about so it is easier to use the SIB and I get to spend more time on the water.
I would recommend keeping it inflated rather than folded, especially if it is in a cold environment. I felt that as the fabric was very cold and stiff (I use it in winter) it could cause problems if it was constantly being folded.
As for the pressure, when it's solid then it's ready! I was told that the stirrup pump adaptor will 'pop off' at the correct pressure. I always go a couple of pumps over that. As you say the QS has an 'airdeck' and that needs to be at a much higher pressure. Once again when it goes rigid then it's done.
The SIB now lives outside all year, (upturned on the trailer to keep the water out) and just gets pumped up a bit when I'm about to use it.
I live north of Inverness so if you fancy a SIB expedition Brian, give me a call.
Best of luck.
Keith (whatever happened to Crazyhorse) Hart