First of all, I try to follow local customs on tipping which, BTW, do NOT imply that it is the norm to tip housekeepers in the USA for ordinary service despite what Marriott would now like us to believe.

In Asia, as a general rule I do not tip.

I also do not tip, except for something extraordinary, in properties that impose a fixed 10-15% service charge on the room rate or entire bill.

Similarly, I normally don't add a tip in restaurants that impose a fixed service charge in the USA, either for everyone or for larger groups. Even in cases where I otherwise would have tipped more, when they go to the fixed rate, they don't benefit on the upside from me. In Europe, I follow the local custom, from no additional tip in restaurants to rounding up to 10% in some southern European locations.

In the USA, most room service bills include a delivery fee (flat amount) and a service charge (18-20%), so I do not tip beyond this unless something truly special was involved.

I was taught to tip valet parkers only when the car is returned. In most places and in decent weather, $2 seems enough unless the car has been washed, had windows cleaned, or has obviously been preheated or precooled for me. I tip more in bad weather and in certain locations but local valet parkers seem pleasantly surprised to even get a dollar.

Housekeeping? Despite Marriott statements, it's not a social norm to tip. I tip for special or personal service, such as delivery of forgotten toiletry items, and I would tip if I made a big mess, but cleaning rooms--especially between guests on one night stays--is done to benefit the hotel (so that the health authorities allow it to continue to operate) and is not a personal service to the guest.

I rarely use bell staff--I can manage wheeled bags and doing it myself is faster--but when I do, I usually tip at least $5 or the local equivalent. My employer will only reimburse $1 for a bag, which I consider to be embarrassingly low in most locations, and I'm not willing to "cheat" on this, so I avoid using the service when possible.

I generally ignore self-serving guides that always seem to overestimate when making claims about what the standards are for tips. Some of these are laughable, including a lot of information in travel books that suggest far greater tips for foreign tourists than civilized/sophisticated/educated locals would do.