I totally agree. Me personally I have Bell Calibrate all of my measure tools when they are due to be calibrated. They do that service for me since I use them at work on the regular. Its quite important as you said to keep the tools clean as well. I usually will take a piece of paper towel or cloth and dab it with some acetone and clean the tool up to ensure I have everything set.
It is quite important though to ensure you have both the tools calibrated and cleaned. You bring up a very good point. Its always important to keep the tools covered and put away when not in use.