I would wash it. I learned my lesson just recently. I donated a quilt to a local group for fund raising. It was being hung for display and someone noticed a rosy smudge' on the flannel backing and thank goodness it was called to my attention so I could fix it. I had washed the quilt and one small red triangle right in the middle of the quilt bled..........it was the only one. (This was a scrap quilt.) I brought it home, took out the block that had the problem and put in a new one. But the rosy smudge was still on the yellow flannel backing. I tried several things and couldn't get it out. It was just about exactly in the center of the backing, so I got one of my orphan blocks and sewed it on.
I was really grateful to the person who noticed it and told me about it so I could fix it. It will be raffled this Saturday.