I had a couple of very intense storms come through the Carolinas last summer. My hops did pretty well in part because of the way I designed my above around box.
I built the box with a bar going over it that the line was attached to. Between the bar and the top of the trellis I kept the line snug and had strings going out for lateral support.
I took a bunch of photos through the season to document things, and because I was obsessed! Check out the first few photos to see the bar design: http://https://picasaweb.google.com/john.marsters/HopsTimeline?authuser=0&feat=directlink
So the string didn't put pressure on the plant, in fact under the bar the bines themselves were slack. If I didn't design it with that bar and keep it tight I think the dramatic swings would have yanked pretty hard on my first year crowns during one specific storm and may have uprooted them.
I'm glad you posted this. I'm about to go to Lowes to get some anchors for my new setup and I hadn't thought to incorporate these ideas into my new design!
Also, if you have limited space like I do keep up a good fertilizer regiment so the plants don't rob eachother of nutrients!