Not quirks, but common sense. I've one of those plastic things to use on toothpaste tubes (and similar) to get the last bits out, usually enough for several more brushes. For shampoo and shower gel and similar bottles, I upturn them so the liquid flows to the "top" so can get the dregs out. For "pump action" things like hand cream, the pump doesn't suck out from all the edges/bottom etc so I get a sharp knife and cut the bottle open so I can scoop out the cream that's lodged around the sides and bottom.
When going on holiday, I take "just enough" shower gel, shampoo, hand cream, etc., for how many days away. I always mark the bottles with usage, so I can see how much it goes down, say weekly, and then when it's one/two weeks from the bottom, I put aside to pack. That way I never take too much (meaning some thrown away) or too little (meaning have to buy at expensive prices).
I just abhor waste and hate spending more money that I need to. That's from a childhood where every penny mattered. I don't "need" to be so "tight" these days, but it's an ingrained habit from years when my parents, and then, I, did have to count every penny. You never lose that habit.