It depends on your employment contract and the law in your state/country. It also depends greatly on the area your startup's in and whether your startup is even remotely similar to what your employer is doing and whether you might become their competition down the line.

A chance of getting entangled in a lawsuit increases exponentially if you're going to take on your employer. Also, most emp contracts usually also state that you cannot contact your employer's customers or people you met through the company's business engagements for at least a year and usually more. Also, don't try to poach current employees or you'll get sued. Never use employer's computers or any other equipment that belongs to them for working on your startup (that includes take-home laptop that they might have given you). Don't even use their Word or Outlook for your own stuff. Delineate your startup as much as possible and don't share your ideas/prototypes with anyone at work since they might testify against you when called in front of a judge.

I know of one entrepreneur who got sued by a former employer and the argument they used was that even though he quit before he started the company, the IDEA was conceived while he was working for them and that he could not have come up with the idea had he not worked for them. Needless to say, the charge was ludicrous but it diverted his focus off startup and it also scared some investors and the startup was stillborn.