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

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.
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.
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.