Actually there is a number under which you don't have to file. I can't remember what it is but for some reason $400 is popping in my brain. Of course that is from all sources. Also I am not a tax advisor so hey I could be wrong!
There are reasons that a body stays in motion
At the moment only demons come to mind
The need to file depends on filling status and earnings. It also depends on withholding and whether you want to get it back. IRS.gov has a nice interactive feature which helps you decide whether to file.