According to this answer, parfocal lenses (lenses that can zoom in/out without losing focus) are relatively rare and more expensive. However, the original poster's kit-lens is parfocal. ...
Kerri Shotts says (emphasis mine): Also, if you already have great glass, there is little reason to get more (unless you need it for a certain reason). E.g., if you have a 70-200 f/4, don't rush ...