Tours are prepaid so you'll only need money for drinks, laundry, souvenirs and tips; for this, bring cash. There are ATMs in most main towns, but it would be wise not to rely entirely on being able to use plastic. Credit cards are accepted in some hotels and souvenir shops, but only in major cities or well-touristed areas.
Tour guides You will usually be accompanied throughout your visit to Bhutan by the same tour guide and probably the same driver. Though it's against the official TCB policy, these people expect a tip at the end of the trip. Many leaders on group tours take up a collection at the conclusion of the trip and hand it over in one packet. With a large group this can be a substantial amount and the practice has created high expectations on the part of Bhutanese guides. If arranging tips yourself, hand them over in individual envelopes the evening before you leave, as things get rushed and easily forgotten on the day of departure.
Trekking guides If you've been trekking, it's appropriate to tip the guide, cook and waiter. Horsemen also expect tips, but this can be minimal if they are the owners of the horses or yaks and are making money by hiring out their animals. The stakes go up, however, if they have been especially helpful with camp chores and on the trail.