If I'm just making tea I put in however many cups I need. I'd only fill it if I'm boiling water to do the washing up or something and actually need a whole kettle full. At one of my old workplaces it was expected that you both refill and reboil the kettle immediately after using it. I don't really know what the point of that was since you'd need to boil it again when you actually wanted to use it.
Only fill to what I need except if I want cold water. My dentist told me years ago not to buy bottled water but rather boil the water and let cool, then refrigerate so I've always got clean, cold water on hand.