I think it depends on when you plan on shaving. If the lotion isn't all the way dry yet and you shave, you'll probably get some fading. If it's hours after it's developed I'm sure you'll be fine.
Try and shave before you apply your tanner next time, or if you forget, do what I do. I keep a gradual tanner on hand (Jergens, etc) and apply that on the areas you shaved. Helps a ton, and you don't get buildup that can occur by reapplying a full strength self tanner. Since shaving doesn't take a lot of dead skin off, a gradual tanner is usually all you need