Tipping is not part of the culture in Myanmar. However, by now they have had enough exposure to visitors who do tip that some may have come to expect it. These people are very poor, and we felt that tipping was a way to distribute what we were spending. For waiters, luggage porters, maids, bus drivers, taxi drivers and tour guide gratuities are a nice addition to their meager wages.
It depends on how long you use the car/driver and the guide. And of course it depends on how satisfied you are and besides that, how much money you can spend comfortably since this is different for everybody. Do keep in mind that drivers usually rent the car from a car owner and that from all the money they get for a trip, they have to pay the gas, toll, maintenance (no matter how costly) and all other car-related things.
Tipping is not compulsory indeed but you should reward your guide(s) and driver(s) if they do their jobs excellently. Normally, it is $5/day for guide and $2/day for driver.