I've always just used estimates because I want to get my FAFSA in early, and often forget if I wait past turning in taxes.
Submitting a change to FAFSA, should be taxes be different, takes about 3-4 minutes and is no big deal. Just log on, change the number, verify your information, and send it.
Either way it isn't hard. My recommendation is just to get it done while you are thinking about it, that way you can't forget! Starting a new application and filling it out only takes 15-45 minutes, depending on how familiar you are with the questions and what connection speed you have.