Is jail breaking an iPhone bad?

Is jail breaking your iPhone bad

Yes It voids the warranty

No, it isn't. It allows you to do many cool things that otherwise you couldn't do with regular iOS software provided by Apple. However if you mess with it in a way you don't know what you're doing, then yes. Which is why Apple frowns upon it. But Overall jailbreaking should be left to experienced technicians and those who understand what they are doing with their iDevice. And it doesn't void the warranty as long as you restore it from iTunes before you take it to Apple.