1. If I'm going to ever be back or play on upselling more work, I'll make my life easier with a panel that has some spaces left in it. If it's strictly $$, I'll tend to go as cheap as possible. But before I leave zero spaces, I'll ask the question if they want to spend a few more bucks to have some extra blanks for the future. Usually they want to go up just a tad to save ripping out a new install to replace it when the whirlpool tub goes on sale at the big box store.
2. Same as #1. If I have to move a breaker from the main panel to the sub, it makes life easier if they're the same ones.
3. Generally I'll try to line up the tops of the tubs. If I'm mounting one below the other, I'll line up the centerlines unless there's a wall of something that make me rather line up one of the sides instead.
