Difference between tepid water and lukewarm water

Pizza Dough is usually a yeast bread so you want your water to be warm (100-110F) to activate the yeast without killing it.

Quiche crust, like pie crust, develops the best texture when the flour is mixed with the butter (or shortening) in a solid state, so you want to add other liquids at a cool temperature to prevent melting the butter to a liquid and creating a less flaky crust.

I am not really sure about tortillas, but with the first two examples you can see that temperature is important and differs from application to application.

I too had never heard of this until I read @user5561's answer. So although it's news to me, I'm going to venture an answer. I believe the "reasoning" is as follows:

The overall taste of the whisky will be comprised of the flavour of the water, the flavour compounds generated by the fermentation and distillation, and finally the flavour generated by aging the whisky. Therefore, if you wish to water the whisky down, you should use the same water the distillery used so that you are not adding additional flavour components to the final drink. If you use a different water, you will be introducing flavours that weren't present in the whisky as it came out of the bottle.

Now there are three assumptions this belief will be predicated on:

1. There is a human being on this planet with a sense of smell so developed that they would notice the difference.
2. The water you are using will somehow always make the drink taste worse and never complement what's already there.
3. The original flavour components in the water the distillery used have not undergone any changes during the fermentation, distillation and aging.

I take issue with the first two assumptions. You won't be able to tell the difference unless you're using water you just scooped out of a swamp and, with good water, there's no reason to believe it could not possibly improve the drink. I doubt the third but don't have any evidence either way.

I've come across a similar argument used for pizza, coffee, stock, etc... For example, since coffee is mostly water it's the most important ingredient. It seems logical until you realize that the neutral flavour of water is so subtle that it's quickly overwhelmed by just about anything you add to it (especially ground coffee beans).