Publishing for Flash 7 or 8 player

I’m a relative newbie to Flash; whilst I can understand that if I publish my site for flash 8 it will not display properly on browsers with flash 7 players installed I was not expecting the reverse - ie that it would also be faulty on browsers with Flash 8 player if I publish it for flash 7. Is this a common problem or does it suggest that that there is something wrong with my file? What is best practice in this case, to publish for 8 and include a detection and redirect script for those who do not have the latest player installed on their browsers? Any advice would be much appreciated as my client thinks I’m an idiot.