Your horse needs to have an full examination done at least twice a year, many times there will be no need for any dental work but an examination needs to be completed to determine the need. One of the most important aspects of equine dentistry is the use of a full mouth speculum shown here –
This device is absolutely necessary to be complete in the examination and makes it much easier for the veterinarian or dentist to complete any dental work that may be needed, especially in the very back of the mouth at the last molars. I have opened many of horses mouths that have very good balance in the front of the mouth up and including the first couple of molars only to find in the very back two very large spikes protruding upwards on the last molars because no one really opened the mouth using a mouth speculum.
So be sure that your horses mouth is being looked at very carefully and the proper tools are being used to perform the necessary examination and work. Small adjustments to the teeth in your horses mouth can have dramatic effect on their movement under saddle. If a horse has small enamel points or worse large spikes in their mouth they will not be comfortable and this can cause head tossing or even a slight positioning difference in the head. This tossing or head position can cause problems with the bit, or the overall smoothness of the ride. In fact one of the first things you should do if your horse is tossing his head while you are riding is to have a dental examination. Twice a year examination and work usually prevents any of this from occurring.