1. Chop.
2. Glue and Vise.
3. Underpin.
At a previous job, I used to skip the vise, but I found that my corners weren't as tight on small frames (1/2").
I also noticed that some corners didn't hold after the underpin, and I'd have to put a vise on every corner. This would use all of my vises, so I'd have to wait for them all to set before nailing more corners.
I also hated cleaning up the glue that got on and in the underpinner.
This was in a woodshop outside of the store where we finished the frame job.
Now I have all of the equipment in the store, and can let corners set while I fit a different project.
Brian