Is it a ‘thank you for prepping my room’ or ‘please clean my room today’? If you tip post cleaning, it’s likely going to someone else the next day. Many hotels now only do housekeeping on demand. How do employees feel about this - do they miss the tips or are they happy for a less stressful workday?
ETA- I’m in the US. Does the rest of the world tip housekeeping? I always have when traveling because I do at home, but I don’t know what the norm is.
The reason why restaurant tipping is usually percentage based is because the level of service expected scales with the price of the items on the menu. The expectations on servers in fine-dining is a lot higher than a neighbourhood pub and so is the price.