Sometimes it is best to pull some out and not use them for either light or dark. My strings are narrow, so it's not as big of a deal and I have a huge variety. However I'm pretty sure I've got a few fabrics that ended up in both the light and dark piles.
I think one consideration - if you have a lot of a type and it's going to be showing up often, then it may work. However if you have mostly very light and then toss in only a few questionable lights, they are going to stand out like possible mistakes. Though sometimes that look is fine too. All depends on what you can live with.