I've had very good results with washing hair with tinsel in it, it never did any harm at all, and also with using a small, lower temperature straightening iron (the kind offered as a travel version) for straightening tinsel. It only works well if the tinsel is straightened one by one though. Just never ever apply heat to plastic tinsel. Colored tinsel might lose its color partially in the process though if it is very crincled. I once had that happen with a Princess Sunbeam. Princess Royal Blue was totally fine so I think it has not been colour related, just the fact that extreme crincling seems to harm the painted surface of the metallic tinsel at some point.