I've ridden 172.5 cranks for 3 years. My trailer pulling bike, the basanova that I pull my trailer with, has 175s. I just started riding 170s on the havocstaff for fun, I didn't notice much of a difference racing or training, but when I went to go for a 2 hour ride while pulling the 100lb. trailer the crank length was truly noticeable. I'll be going back to 172.5 soon.
Pulling a trailer is something I am not sure I should be doing. Mostly because it is really hard. It is basically no-stop motopacing for as long as you are pedaling. It has to be the closest thing to motopacing without actually doing it. When you pace, you are pedaling super hard, at 30-40mph, but you feel wind like you are going 10mph. When you are hauling something, like my two boys, you are going 13-17mph, and feel little wind. Your pedaling like your going 30mph but you feel wind like you are going 13.
Reason I am not sure if I should be doing is mostly because my legs feel shattered after a 90 minute ride. Its unreal, like you were lifting in the gym for a long time.
I'm starting to find descent roads around, its really flat, so pulling a trailer isn't nearly as hard as it would be had I was riding in KC. Finally starting to find places to get a good 1 or 2 minute interval in, some false flats, small inclines, etc.