After having trailer tires spin balanced over the years I've always wondered why it is some seem to take large weights to complete the balance and some seem to have a wobble to them as they are spun.  After getting our new 5th wheel last October we towed it about 1000 tow miles, then took it to a major tire shop to have the tires spin balanced.  After watching the first couple tires be balanced I noticed the operator seemed to be having problems balancing the tires.  I asked if there was a problem and was told the rims were bent and causing problems to balance.  As the tires/rims had only 1000 miles on them how could they be that bad??  Now these were the white steel wagon style wheels  most trailers come with.  I decided to call the rim manufacturer and see about a replacement of the bent rims.  I was able to talk with an engineer at that time.  He told me he thought the balancing had been done incorrectly.  I was told there steel wheels are to be balanced using a bolt hole plate, this is a plate with studs that bolt to your wheel using the stud holes in the wheels, then the wheel with the adapter is put on the balancer and balanced.  The common way to do a spin balance is to mount the wheel to the balancer using the Pilot hole in the center of the wheel (this is the large center hole in the wheel). I was told the Pilot hole could be off center as much as 1/8".   I took my trailer back to the same tire company and told them of my findings.  On this type wheel they had never heard of using this type balance, but would do it.  What a learning experience it was for both myself and the shop crew.  Watching the spinning tire using the bolt hole plate there was very little wobble if any and much less weight used to balance the tires.  Example,  one wheel had previously showed a lot of wobble and took 22 oz to balance and now took 5 oz and showed nearly no wobble. I have seen a lot of talk on forums about bent wheels over the years, now I know the possible reason for some.  If your going to have your trailer tires balanced, I suggest checking with the wheel manufacture as to there requirements for balancing there wheels.  Added note: The best way to have the tires balanced would be to have them spin balanced on the trailer.  Doing this balances the tire, wheel and the drum, but it's getting harder to find tire shops that can do this type of balance.