I am making macrons and I have a few egg yolks leftover, I don't have time to use them up tonight and they've been sitting in the fridge 5 days now... should I toss or can I use them still?
They are likely at about the end of their useful life. Fortunately, it doesn't sound as though you have too many, so it probably won't be a big loss to toss them. For future reference, both egg whites and egg yolks can easily be frozen. In the case of yolks, be sure to whisk them together before freezing them. If you don't, the membrane surrounding them tends to toughen during freezing and form annoying clots when you do use them.
I use the 5 day 3 day rule. 5 days for things that are raw, 3 days for things that are cooked. I certainly agree with Cynthia, though I'd err on the side of caution and toss 'em.