In my previous work we had a few cases where drivers running low riding wag’n’drag transporter drags failed to raise the suspension after they dropped it during (un)loading, loaded especially running with suspension fully lowered would knacker the air bags in short order.
So i suggest it’s a safer bet to leave it in a raised position (maybe not so good if you are only just clearing a bridge at normal height though 
) than to run with it fully dropped, though maybe that depends on design, those low riding wag’n’drags had really small air bags which presumably would get damaged when deflated over road shocks.