Do it if you are sure you are going to use it. If there is a possibility that you will not, I might stick with taking the dependent care credit when you file your taxes, which is helpful, but not as good as the FSA.
I do believe you can also delay your decision to contribute to an FSA until your kid is born. I think this is a "qualifying event" and allows you to make changes to your benefit elections within something like 30 days of baby's arrival. You'd change your election along with adding baby to your health insurance plan.
Remember, you can only be reimbursed for what you have already paid into the FSA account, and for services actually rendered. For example, if I pay a semester's preschool tuition of $4K in August for the period of preschool from September to December, and I have $2K in my FSA at the time, when September concludes, the FSA will pay me $1K. If I only have $500 in my FSA when September concludes, the FSA will pay me $500. I know these numbers don't make much sense timing wise, but hopefully you get the picture.
For extra mustachian points, I like to pretend that the FSA money doesn't exist, pay the tuition like it is a regular part of my budget, and when the reimbursement is available, I put that directly into a savings vehicle.