I'm trying to decide if my gf should apply for a 10 year multiple entry or just a single entry- does choosing the 10 year hurt your chances by implying long-term plans to the officer? or do they always mark it down to single entry and only give the 10 year after she's returned and applied again?

There is no choice when making the application. It depends upon what they feel she should be issued.

For a first time visit it most likely will be only a 6 month visa.

Link to application form: https://evisaforms.state.gov/ds156.asp

For info on the application process see this page on the embassy website.

http://bangkok.usembassy.gov/non-immigrant_visas.html