Is it usually just the posterior sides of teeth that are affected by acid reflux or both? could one episode of reflux cause noticeable wear?

Yes: Yes, it is the posterior side of the front teeth especially that are affected by acid. Typically it is from bulimia. Just having acid reflux, shouldn't have acid getting all the way up to your mouth and teeth.

No: You would have to have very prolonged exposure to acid reflux or a purging disorder to cause dental issues. All tooth surfaces could be effected.

All surfaces: All the teeth can be affected by acid reflux. You need a thorough cleaning and diligent home care- possibly fluoride and/or plaque rinses to help save the enamel.