Should You Tell Your Manager/Boss/Employer About Your Startup In Stealth Mode?

There are quite a few engineers who work on startups in their off-work hours using their own resources [email, computers, Internet etc.], while being full time employed. In such a case, what should be the strategy? Should you tell your manager, boss about the startup you're working on?

In my opinion, as long as you're not in a competing business with your employer, there shouldn't be any problem. It'd also help to read out your company policies. I'd like to know views of engineers who're working on their own startups in part time.