Do you need a flight out of Thailand for a Tourist Visa, or is a bus ride to Laos acceptable?

To obtain a Tourist Visa for Thailand, some consulates require proof of onward travel, which could include a flight or a bus ticket to another country. For example, the Washington DC consulate asked for this proof, while others may not. A bus ticket to Laos should suffice, provided it shows travel to a destination outside Thailand and not just to the border.