Trailer Tires

A couple of observations, I've been towing stuff since about 1975. I started hauling my cars on used street tires on open trailers. Had my share of blow-outs, always carried two spares. Finally started buying trailer tires about 20 years ago. One of my concerns is rim width, it seems like real trailer wheels for 3500 lb axles are usually 5" wide. If you pump tires on these rims up to max pressure, you'll probably have tires that'll wear more in the center. I wonder why a 6" rim isn't used?
Now about running 225/75-15 tires when 205/75s are standard, please check and make sure they won't rub inside or outside of the fender! On two of my open trailers there isn't much room on the inside so I'd have to run a spacer. Also if using a 5" rim width, what tire pressure will give you even tire wear? If you run these tires please let me know what pressure you're running in case I'm ever tempted to try this size.
Looking forward to hearing from anyone doing this with 5" rims.