In order to see light ... there must be dark.
In otherwords, value is relative to what it is adjacent to.
I'm also in the middle of a scrappy quilt and yes, most of my stash is "medium" - and there are also fabrics that can be both depending on how/where you cut it.
I keep three piles ... light, dark and medium.
Sometimes I will use the medium as a light, and sometimes as a dark - depending on what I put next to it. If my fingers next touch a black square - I'm grabing a medium color to use as a light. And visa versa.
Bright mediums can be the trickiest so I use them sparingly in an "anything goes" quilt and try to save them for a "bright scrappy" quilt pairing them with either white or black.