Hi ,
In my experience with 18thc and earlier tankards etc that have the often found handle/body cracks, annealing related or damage related,both are affected quite considerably with regard to value.I can also agree that they can grow if the item is used,though it is now possible to have some cracks stabilized by leaching into the cracks very thin uv reactive glues,this does involve a little flexing of the area around the crack so is a potentially disastrous procedure,so far :X: this has not happened to the few i have had done this way.
cheers ,
Peter.