I remember there being SNFO standards and NFO standards and the NFO standards didn't apply until I was winged, this was some time ago though so... And yes, it was actually a worry for me since my eyeglass prescription was a little over the limit for SNFO at the time, I passed NAMI but sweated it a little until I was winged. They only occasionally checked my eyeglass prescription again though and just asked me to do the eye chart with and without glasses after that, but I was an NFO though.
I'll pass some advice along that I got from a former Skipper who had bad eyes after I got burned by a Navy ophthalmologist when I was in my first squadron, if you have concerns about how bad your vision is get it checked out in town before going through the Navy. Any halfway decent optometrist can give you a quick check to see where you are, a small price out of pocket for some peace of mind.