CK,
You are correct has the horses teeth been done recently and make sure they check for wolf teeth. The majority of head issues are related to the mouth, some horses may need their teeth done more often than others it just depends on how sensitive they are.

The only other suggestions I have are:

1) Your saddle fit. We had a mare that had been with the trainer and when my husband went to ride her at the trainers house before we brought her home she would throw her head and back up. We started looking at things to see what was causing the problem and realized that the saddle my husband had pinched her withers something terrible. We purchased a new saddle for her and now she is fine.

2) Your horses feet. They may not be lame to look at but may have shoes that are too small and pinching the foot. I had a client that had swithed farriers and the new one put to small of a shoe back on the horse. When she would go riding the horse was constently tossing his head. It took us a little bit to figure this one out but when we did and put the larger shoe back on the horse was fine.

Once you correct any potential problems, teeth, saddle, feet, etc. It may take a while for the horse to realize that it no longer hurts. Especailly if it was a mouth or foot problem since it will take some time for the bruising (foot) or sores (mouth) to heal. If they continue to do this after a couple of weeks (if you are riding regularlly may take longer if you only ride weekends) than it can be considered a learned behavior (Cause from the discomfort) and may require some retraining using a method that you prefer. Tiedowns are fine and used properly do there job and help the horse (barrel & roping horses) but I don't believe they should be used as a quick fix, you need to figure out what is causing the problem. Sometimes this may even require getting your vet involved.

Hope this helps.