Fiona - be careful. I got hauled down to EXT once and berated for a supposed pattern in my sick days. Now fortunately for me I had doctors certificates for every day I was sick - but they do have software that looks for patterns in sick days off. It mainly catches CAs.
I was able to extend the digit to our ex-FM, but if you don't have the certificates, you could find yourself in the crap.
If you do take extra days - and I completely agree that the poor rostering will eventually force the hand of anyone who actually likes their family - go and get a note from your doc. I always do, no matter what the problem. It is the only way to be safe (well reasonably safe).
I can honestly say that I never actually took a day off when I wasn't sick - but by the same token, I refuse to fly when I have a cold. Colds are very common in winter. They hang around, and is most unwise to go back to work too early...