The same principles apply for a medical flexible spending account that you need to figure out how much you will definitely spend before committing to how much will be placed in the account. Any amount not spent is forfeited,
This isn't true anymore. You're now allowed to roll over a certain amount of it per year. I believe that's at your employer's discretion, and is only for the medical FSA. My employer allows us to roll over $500, I believe.