A large temp drop can be fine for some fish (e.g. Uruguayan species) but stressful for species that live along the equator, or other temp stable environments.
When I lived in Wisconsin there could often be more than a 40'F difference between my tanks temp and the tap.
So in winter I always mixed heated water with the cold, so not stress out the more sensitive species.
If you are worried about the substances that build up in water heaters, (in the US at least) all water heaters have a spigot to near the bottom of the unit to drain out sludge into a container. If this is regularly done, no problem (I had none mixing hot with cold water)
Every so often I'd drain of that sludge until the water felt un-viscous. Even without fish to worry about is is good to drain out sludge a couple times per year (and is often suggested in the trouble shooting part of the water heaters instructions.