Should I loot every house in Witcher 3?

Well, if nothing else you can find vital items like Alcohest, Dwarven Spirit, food and drink without having to buy them. I don't think there are any quests to be found outside of looting things that are obviously 'treasure chests'.

Depends what you're looking for. The chest contents are randomized, and you can pick up expensive crafting components or recipees/blueprints even in shabby village cottages. So it's not a complete waste of time.

That said, you'll get better loot looking for hidden treasure or monster nests, and I don't remember any really unique content you might miss if you skip on being a kleptomaniac.

I'm the same way, it's that oldschool game treasure hunting mentality, it's tough to shake. Anyway, I can say that while in the base game I can't think of anything that was extraordinary in a random NPC house. As mentioned above, I can say I have never needed to craft or purchase Dwarven spirit or strong alcohol needed to replenish stocks after meditating. Also, you do come across some places that have a number of furs that can land some decent coin at the right merchants.

Now, I just started the first DLC (Hearts of Stone) and it's a whole different ball game for random treasure hunting in the areas that are newly created on the map. I find awesome weapons and armor all over the place. While this isn't so much in people's homes, it definitely is if you go off the path a little. Particularly, a light house in the far northwest corner above Novigrad, there are some random people living there and I almost didn't go in. I'm glad I did, because the all black New Moon armor is sweet and way better than what is provided during the story.