To check for level you need to measure the TT frame at the front and rear. How do you know for sure if your road is level? You want to be parallel to the ground.  The spring bars don't have to be perfectly parallel to the frame. 
However I would try and not go any more out of level than one link with the spring bars. 
On our last TT the coupler was mounted on the bottom of the tongue. The tongue was made from 6" tubing. When I hooked up the spring bars and tried to keep them level I had to be on the last link. The main reason to keep the spring bars as close to parallel with the tongue is so that you don't put them in a bind when turning tight corners. Having them 1 link high out of level won't hurt anything.
Look at it this way. The ends of the spring bars are like the end of a pendulum. They need to be able to swing back and forth on the chain. If they are to high they won't have enough room to swing. I've seen some guys spring bar ends clear up next to the snap up brackets. I can't imagine how much stress that's putting on all the WD items.