I heard best position for jaw if tmd is open. is that like a smile?

Not true: If you have tmd, the best position for your jaw is its most relaxed position. This is usually with the teeth slightly apart, but definitely not open wide enough for a smile, and definitely not closed all the way or with the teeth clenched.

Depends: Depends on reason for tmd. If you are hitting on your front teeth first when you go to close on your back teeth, then open like a smile is ok. But in that case you need some type of treatment such as a bite adjustment.

Slightly apart: The rest position is lips together teeth apart with the tip of the tongue resting behind the lower front teeth.

Depends: Each individual case may vary including the best position that the mandible needs to be in. See a dentist to help.

Teeth apart: Teeth just apart, in a comfortable position. Should not be touching at rest.