Obviously, when sick or not fit to fly, don't hesitate to call in sick.
When considering how often to call in sick or whether or not one should use MUS/MUD, consider your age and how many years you have for the company to contribute to your 401K. $15,000-$17,000/per year into one's 401K beginning around the age of 50 will compound nicely.
Once you reach 60 or so, consider another strategy to manage your sick bank.