Here is my take on this issue: If it is better to drink wheatgrass juice on an empty stomach, then doesn't drinking it with other juices go against that rule? The stomach can absorb the nutrients in wheatgrass juice easier if there is nothing else in there to interact with it.
Many smoothie recipes call for a shot of wheatgrass, and juice bars will readily add wheatgrass to whatever shake or juice you are drinking. I guess it is a matter of whether you can stand the taste of wheatgrass juice enough to drink it alone. If you happen to dislike the taste of it, I guess wheatgrass juice mixed with other things is better than no wheatgrass at all! Fortunately, I like the taste of wheatgrass juice, and am happy to drink it all by itself.