When you want to mix two liquids in cooking, when does it matter to make sure you mix Liquid A into Liquid B and not the other way around?

I know that when you are making a creamy tomato mixture, it’s important to pour the tomato into the cream sauce. Otherwise the milk will curdle. But my dad insists that you should also make sure to pour a hot mixture into a cold mixture because the taste will be better that way. I don’t think that’s true. Anyone else here it? What are some other mixtures that it’s vital to pour the first into the second, but not the other way around?