I decided a few months ago that I would always format my memory card after unloading my images from the camera. I happy to say that I've actually stuck with it. I've never had a problem with my memory cards, but I gained the understanding that it's not a bad idea to format the card after each photo shoot. It takes a few seconds of your time, and it helps to ensure that you'll be less likely to have issues with your memory card in the field. Like I said, I've stuck with this habit since I wrote the article titled “Clearing Out Your Memory Card“. If you're not convinced, give it a read. Don't argue, just do it.