some people do that, but if you register any blood in your rig then it will clot, and when you go to bang it you could possibly have a stroke from the clotted blood in the syringe.
also, this is a harm reduction site, and you should always use a fresh unused syringe EVERY time you shoot.
also leaving dope in a rig prepped gives it the possibility in growing bacteria or something from the water used not being sterile, something in the dope, or just b/c it's sitting in the right temp to promote bacterial growth.