When you're applying for a corporate job, it makes sense to go for a corporate look at your interview. You want to look like you can fit in and do the job. When you're applying for a job at a startup, deciding what to wear isn't so straightforward. Startups are the antithesis of corporate, and without a dress code, their employees usually wear pretty much whatever they want. But that doesn't mean you can wear whatever you want to your interview.
Every startup is different. To some extent, you need to use common sense based on what you've heard about this specific startup and the specific role you're interviewing for. Sales jobs and other jobs that require meeting with customers may require a more pulled-together appearance. So what follows, then, aren't rules but guidelines that will help you decide how to present yourself.
It doesn't matter how casual the dress code is; your clothes should be freshly laundered. You should not be rumpled, smelly or stained, it will seem like you aren't able to handle basic self care, or give the position your respect.
A pulled-together look is always better. That doesn't mean formal, but it does mean looking like you made an effort. It's not so much the specifics of what you wear, but that you give an impression that the interview is a big deal to you.
Not all casual environments are the same. Some are fine with short; others draw the line at jeans. It's better to be one step more formal than the workplace than to push it too far.
For both gents and ladies, khakis and a button-down shirt are always a safe choice. There's no need for a tie. Ladies can also wear a dress; just don't go too short or too low cut. If you go with jeans, darker colors read more formal than light colors. A sports jacket or blazer also step up your style when you wear jeans.
No smart company will make the decision to hire you based solely on what you wear to the interview, but it's another piece of information they'll consider. Miss the mark too badly and they'll wonder about your judgment. Make a smart wardrobe choice for your interview, and once you're hired, you can dress like you belong there.