I gotta agree with Gator about trucks and dirt. I am a mechanic, and our fleet is mostly local. Hauling mainly dry bulk, some hopper bottom grain, and tankers of De-icer in the winter. All of the drivers are company workers and dont get paid to wash the truck so they could care less. But all the crap that gets caked on over time, plus the mag chloride here in Colorado makes electical/lighting fixes into a nightmare. Can we say bad ground? Corrosion?
Anyway, back to alignments. I do trailer axle alignments in the shop if there is indication of a problem, but power units are sent out to a local company. We base our alignment frequency on tire wear, driver input, and service inspections of the unit. Our frequency for alignments is prolly quite a bit higher than most folks as our facility is a transload yard. We off load product from railcars to trucks for delivery. All that crossing the tracks (yard is gravel not paved) takes a healthy toll on equipment.