Not sure about your area, but George on here would know the local rules best, in Puget sound, I can sail either nfs, or fs. NFS is 18sec a mile slower than my fs time. Altho this can vary from 15 to 24 secs depending upon headsail vs spin size.
Also if you start a series in nfs, and you use a spin for one of 8 races, at least locally, ALL 8 races with be at your fs rating, not 7 nfs, 1 fs. If a single day race, you usually need to say what you will be doing by a day or two before the start, depending upon the length, club etc.
Generally speaking, you should get a fs/nfs rating.
She drives me boat,
I drives me dinghy!