I find psychologically people feel better when they get out one day at midnight and the next night at 9. They can catch a movie, go out to dinner...they don't feel like their whole life is being sunk into the production. Less burned out actors makes for (potentially) easier tech.
But, most theatre's I work need the morning hours to do scenic notes, costumes notes, etc, etc. So, days are either 12:00n - 12:00m, or 2:00p to 12:00m.
I like the two 10/12s together and early to get through more of the show earlier for departments to respond with more time before the first audience.
And, I am always dealing more with long term fatigue . . . form a tech week into a week (or more) of previews - so, get them done early, together, and get more into the time schedule we are going to be in for previews . . .