A standard board is fine, especially if you have the bag bellows. If you don't, the bag bellows may end up being cheaper than the recessed board. It all depends on how much movement you need - are you shooting architecture with it, or just landscapes?
If you don't want to get the bag bellows, get the recessed board. I couldn't get adequate rise and still lock down the front standard w/o really cranking on the knobs to keep the bellows from pushing the standard around so I got the bag bellows. I also don't like being limited by the camera. The bag bellows will work with everything up to a 210mm and will also fold up with the camera, contrary to what some may say.
vinny
I do very little architectural work -- almost all straight-forward landscapes or similar images.
To be frank, I'm jumping ahead a lot further than my actual knowledge and have a lot to learn about movements, bellows compensation, etc. And, I don't have the bag bellows at present.
Plus different 90mms will have slightly different distances. The Nikon for example is around 98mm IIRC flange distance. That makes it easier to use. So if you haven't bought the lens yet it's something to consider.
My 90mm SA needs a recessed board (I don't have a bag bellows) on my Shen. I can focus it OK with a flat board but movements are highly restricted - in fact rise, fall & shift are not wonderful even with a 15mm recessed board (although tilt is fine) so I think a bag bellows is in my future... Using a recessed board is awkward too. Given a straight choice, and especially if I was doing architecture, I think I'd go with the WA bellows in future.
The stock bellows on my HZX45A-II is fairly stiff. I have not tried a 90mm yet, though I did use a 75mm on a flat board. Where I to do that often, I would get the bag bellows to allow easier movement adjustments.