I think most of you are correct, in that it usually
doesn't make sense to place levelling blocks or boards under only one axle. It also usually
doesn't make sense to place blocks under both sides of the trailer. As always, there are exceptions to everything. However, the instructions with the levelling blocks are probably written for the very newest users, and if someone has no practical experience with levelling or axles & springs, they need to be told what not to do. I believe that is why the instructions are written that way.
Again, I agree that blocks or boards should usually be under one side and both axles. I try to get the bubble right smack in the center (I am a land surveyor, after all