I think the older embroidery threads were much less dye-fast. Manufacturing methods for dyeing have changed considerably over the years.
If you really want to be sure, it's pretty easy to test thread. I would sew some onto a piece of white fabric and then wet the whole piece. I think you will be pleasantly surprised to find that most thread these days does not bleed. You could do this with both embroidery thread and the thread you are using to quilt with.