As long as it's just some and it's the lower/biggest ones going first. That's just the plant moving nutes around so it can keep feeding the buds, etc.. I had the same issues except mine was more severe. If you're really worried post some pics and we can evaluate the situation. I just ended up feeding a little bit extra veg nutes and bought some cal/mag and it sees to be keeping her green.
Also depends on how long you're flowering her for/sativa/indica? Sativa will take longer so you want her to be green as long as possible. Indica I would think no problem at all a little yellowing at this point.
Anything I'm missing?
Post some pics!