I agree with mls, it's not the farrier's job to train a horse that won't stand. I'd never expect mine to do so. If there's an issue, I correct it. I do appreciate one that is patient enough to wait while I handle the issue though. Those that I have misbehave for farrier are generally client horses (mine know better) and I don't think it would be appropriate for me to have a 3rd party doing any training or correction on them.
What I look for, good quality work, ability to do specialty shoeing (sliders), experience with corrective work, has a vet that they work with (preferably my own) shows up on time, returns calls or emails. Bonus points for calling to check in between on those that may have had an issue.
The thing that I was most impressed with about my farrier, when Woodstock was injured, it made farrier work tough (torn tendons) so he took it upon himself (with my approval of course) to coordinate with my vet and they came together so he could be sedated and laid down to do his trim and vet could supervise to insure no further damage.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.