According to
this read, the bar is under tension (not compression) during cornering (which makes sense if you think about the force on the outside wheel during cornering). So, if anything, you'd want to pre-tension it, not pre-compress it...just to answer my own question.
2ndly, there is no discernable play in the quick-release bolts. They are very snug.
The Whiteline bar is not designed so you can simply spin the bar to lengthen and shorten it. You have to uninstall it on one side and spin the end piece that the Q-R bolt slides through to change its length. Perhaps I'll try to shorten it half a turn.