The friend that you spoke with is mistaken when he said that, the difference between the 7 inch and 8 inch will have no effect on the wear pattern. alignment, and proper inflation on the other hand will have a great effect on proper wear. Tire techs in general are not the brightest of the bunch, I had one try to tell me that an entire exhaust had to replaced because of a bad muffler, he could not see a place where the two cam apart. Where a saw makes a very convient place where they come apart.
There are thousands of people running 33-12.5/15 on a 7 inch rim, and when I say thousands, I am under way under in the number.
Run what you want, but if you only listen to the tire techs of the world you will be missing out on a lot of life. A lot of them say that they will only install a stock size tire as well.