Tipping is not a standard practice in Bhutan but there are no strict laws against it too. You may or may not want to tip the driver, porter, bell hop, maid, tour guide or any other service provider depending on how much you were satisfied with the services they provided and how much you enjoyed your trip because of them. It is not compulsory to give out tips in Bhutan and you are the one to decide whether you were really treated and served well or not and whether they are worth rewarding.
Keep away from a sticky social situation by avoiding distribution of pens, pencils or chocolates to children or monks, as this frowned upon in Bhutanese culture. In hotels and restaurants, usually a standard service charge is levied on your bill but if you are really happy with the good service that they gave you, you can offer a tip to service staff. Normally, people pay tips after the work is completed but there are people who tip the staff beforehand to ensure a good service too.
You may tip a small amount to the room service attendant on the very first day of your stay in the hotel to ensure that they give you a better service but do not forget to tip them at the end of the stay, if they have really been attentive to you. Porters can be tipped certain amount per bag that they carry. Tipping driver is not too common but if he has driven you safely and made your drive pleasant for you, you may tip them per day, depending on the distance traveled. You can leave the change as the tip for the waiters at the restaurants.