Marc, to tip the taxi driver, just round up the fare to the nearest five dirhams. Tipping isn't a big deal in Dubai, but I did find this in a blog from someone who is living there:
Waiter /waitress: 10 to 15 per cent of the total bill, if service has been good.
Taxi driver: Round up to the nearest note, if driving was acceptable.
Food delivery man: Round up to nearest note, or AED 5 to 10.
Valet-parking assistant: AED 5 to 10.
Bellboy: AED 5 to 10.
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