Premium Gas Only? [POLL]

I just bought a new car and on the gas lid it says, "Premium Gas Only". We all know Premium gas in more money so I am wondering what happens if I don't use premium gas. Is this a suggestion or a requirement? This may seem like a stupid question but I have heard arguements on both sides. Some of you say that is why it the lid says "only" and some of you say that you have had vehicles that require premium but you always used regular and haven't had any problems. So which is it?