Teacher's Assistant?

How will being a teachers assistant affect my college chances? I’m currently looking at going to extremely competitive schools. I’m in the running for valedictorian and my senior year schedule will be the determining factor. I want to go into chemistry, so being a TA for my chemistry teacher would make sense, especially since his program is rigorous and (as he claims) acknowledged by many local colleges? If I become a TA, I likely won’t be valedictorian, but I could get an outstanding letter of recc… Advise?