You do NOT need to fill out at FAFSA for a TA/RA/GA. They are all considered forms of employment, not aid. (Next year, the income from your TA/RA/GA will contribute to your FAFSA results. I currently make $12,000 / year as a GA and my expected contribution for my education is $2,300 / year which is ridiculous).
However, when you do fill out your taxes, it can never hurt to also fill out a FAFSA: it's free, and you never know if you qualify for school grants. You can always reject the FAFSA offer of loans, but it's useful to know what you would be offered, and what's available to you. Personally, I take a small amount of loans, put it in savings in case of emergency while interest is paid while in school.