My Vet recommends boosters every 2 years.  I did titers 2 years ago for Parvo and Distemper and they were both good.  My Vet says I don't have to do the booster, but he did make a comment that "they may not let them in the hospital" without the boosters.  I haven't come to that yet, so I don't know what will happen.
 
Rabies is 3 yrs by law, and I can't not do it