If you have the 14 -45mm and 45–200mm lens it’s nice to carry both lenses – ‘just in case’ you need them. It’s a good idea however to think about what and where you will be taking photos of, before leaving and fix the most appropriate lens to the camera.
If you are going to an outdoor party, the 45-200mm is a great lens to use. So before you go outside, make sure this lens is already fitted to the camera.
Later on if people move indoors, when you get inside put the 14–45mm lens on straight away.
This means when you want to take a photo, the camera is ready to go and your subjects are not waiting for you to change the lens, or worse that you miss a shot because you are changing lenses.