The last time I ordered room service (years ago), the hotel had a mandatory gratuity, and a line on the check for additinal gratuity. I felt that 22% was enough and did not add any additional. After taking the check and starting to walk back to the elevator, the courier loudly said "f*ing stiff" 'under his breath'.

Personally I would never look down on anyone for "only" leaving 22%, but clearly, hotel staffs expect more.