First of all, I am not a vet, so don't take this as professional opinion. I will simply share my experience and that of my clients in the industry based on recommendations by our veterinarians.
It is a good idea to vaccinate all feeder pigs at the age they would normally leave the nursery (8-10 weeks of age). Usually, that is sufficient to avoid any outbreaks before they go to market. In cases of transporting pigs around to shows, it may be advisable to give a booster prior to the withdrawal period to avoid an outbreak induced by additional stress. Otherwise they are normally protected with one shot.
In the case of gilts or boars kept for breeding, they will need a shot of Lepto/Parvo vaccine (and a booster as well) pre-breeding and those vaccines also normally contain erysipelas, so you should be well protected.
These are general recommendations. Your vet may advise otherwise, in which case I would follow his/her advice.