Usually it is just me backing in and I prefer to go in on the drivers side if possible. I pick a few spots that I want to hit with my trailer tires and watch with my head turned out the driver side door window (obviously I am continually looking in other directions too). I find that if I hit the spots I was thinking of I am in with minimal effort. I have even cut a few pieces of 12"x12"x3/4" pieces of plywood and painted them orange for those times it is very dark and need to know those "spots" I am trying to hit (I use them under the leveling jacks too).
To me, if you know your trailer and know where you are backing to, whether its a campsite or 1 mile down a road, always knowing where those driver side trailer tires are simplifies it tremendously.
When I have a spotter, unless its someone who can actually back a trailer up themselves, I find they are only good for making sure I don't hit someone or something.