I have my white shirts that end up with that yellowing underneath the arms. There is a trick to getting rid of it!
Mix 1/2 cup Dawn Dishwashing Soap (original) and 1 cup Hydrogen Peroxide together. Rub it on to the armpit stains and toss into your laundry basket.
Wash as usual with like colors.
The stuff works wonders. I don't use it each and every time I do wash but I will do it every month or so, just to keep things white. I seem to have a crazy number of white shirts and, lo, I wear them all! It also works great on other stubborn stains such as wine and grass. It hasn't bleached or lightened anything I've tried it on so I'm assuming it's color safe. (No guarantees, though. . . I'm refusing to feel guilty if you bleach your favorite top with this stuff so try it out on a spot that you can't see first.)