If the tire has lots of good tread in the middle but the outside near the edge is thin is this ok with the DOT?

I don't know, but change it anyways.

Steer tires? Watch your toe-in and your tire pressure. I run 105 psi on the front.

Steer tires will always wear like that on a live axle, it's just the nature of the design.

They will shut you down if 1 of your grooves is less than recommended for that axle. I have been let go with a warning before. All up to the attitude of you .VS the officer.

Does this also apply on trailer tires??

Yes.

If you have a set back axle and/or run at max gross all the time you might find that 110PSI is what you need to run in the steers 8) .