As a farang one need to keep in mind that what applys to a local, does not apply to a farang.
We need to leave our well groomed egos back home in Europe or the USA (not finger pointing to anybody so please no hard feelings).
To get back to the topic, the farang is considered as the golden goose in Thailand and thus is highly expected to leave a tip. 100 thb should be a appreciated gesture, but do not make it a daily habbit as they will expect it otherwise daily.