I just built that plane and chose to go with the right thrust. It flies just great, of course it is one plane on a short list of planes I have constructed so my opinion probably shouldn't carry too much weight. But it seemed that when I asked that question the general consensus was that it is better to have it. I found the instuction manual only omitted it for sake of ease. It's really not any more complicated than the steps outlined without it.
I had to play with the shim a little bit to get the angle just right. You may want to purchase a one-piece engine mount of the same size in order to simplify the process. I found that the engine got pinched between the two mounts (because one was at an ange and the other is not) and this put a lot of undo stress on the blind not that you epoxy to the firewall. Mine almost became stripped. It all worked out though. How is the plane going? I had a great time building mine.