Desk research against primary sources, with the issuing authority linked where a page states an official fee, rule or deadline.
Government offices and petrol stations are flagged as no-tip settings for a reason: tipping there reads as unusual rather than generous, since neither runs on a service-charge or gratuity model the way hospitality does. Outside those settings, a small cash tip is read as a genuine gesture rather than an obligation, which is part of why cash is preferred over adding it to a card payment that the worker may not see broken out.
The 10–15% restaurant guidance and the smaller round-up amounts for taxis, porters and delivery riders scale with the size of the service rather than following one flat rule, so match the gesture to the job rather than defaulting to the same note for a bag carried upstairs and a full meal served over an evening. Keep small notes on hand specifically for this, since needing change from a large note in the moment is what causes people to skip a tip they meant to give.