I can tell you that flash freezing wheatgrass juice does not destroy it.  Here is the really interesting thing about wheatgrass juice.  Nobody really knows why it is so beneficial to so many people.  As a result, you can't measure the differences between say fresh and flash frozen wheatgrass juice because you don't know what to measure.  The only method that makes any sense is simple observation.

We put a forum in on our website to enable people to record and share their observations after using our wheatgrass juice.  The posts they have generated are really terrific and you can read them here:
http://www.dynamicgreens.com/wheatgrassforum

It is important to note that flash freezing is quite different than just using your freezer.  The reason being that in flash freezing, only small ice crystals form and they do not rupture cell membranes.  This keeps that taste, texture, color and nutrition intack.  While the slow freezing that will occur in your freezer will form large ice crystals and will rupture cell membranes, it may or may not impact the quality of the juice.  Again, you need to be your own best observer.