I would get it done. It's cheap insurance. My mechanic charged me about $80 to do the under body, inside of the doors, hood and trunk. He used to do this at a Honda dealership. He told me that you don't really need to respray every year. Every other year should be fine.
Just make sure that if you do it, try to do it on a day where the forecast is dry for a couple of days. The coating takes a day or two to dry and harden up. If you do it on a day where's there is snow, all that salt and dirt will splash back up and stick to the anti rust coating. If possible, wash your under body and wheel wells first and give it a day to dry before you do the anti-rust. Otherwise, when they spray it on any dirt that is there will be trapped underneath the coating.