Since having the Rangie, I've tried it both ways. On our first run to France with the caravan, I locked the suspension in "normal" mode - the van was roughly level but didn't feel stable at motorway speeds. I checked the noseweight etc. but all was as it should be.
On our second trip to France, I decided to try letting the suspension adjust itself normally. In my opinion, stability at motorway sppeds was much better, with the nose of the van slightly down. (Out of interest, the Caravan Club favour a slight "nose down" over any setup which might be "level" at best or "nose up" at worst).
The latter approach was a good compromise - stable at motorway speeds, but lots of ground clearance for the slower stuff around towns/cities with their spped humps etc.