as noted by PaulaM, it is possible that filing married- separately is better than joint; it's the less than usual case, but it can occur.
For example, one person doesn't make that much in wages, but has very high medical bills that exceed 10% of the AGI,. The medical bills will be deductible (beyond 10%) where if there was a joint return, it wouldn't exceed 10%.
There are downsides when it comes to state and local taxes (SALT) as each can only take $5000 and there are downsides in man y other situations.
Trusting TT to figure it out for you is the way to go; I wouldn't do this manually as the risk of error is way too high as the rules can be quite complex, depending on your personal situation.
