i find a lot of the time you can figure out what direction your opponent feels the most comfortable continuing the TAC combo with...for instance if they like to go for infinites.
That's just at lower level play. I used to TAC up with Dormammu every time because I could only complete his TAC while facing the same direction that way. This is because for all of Vanilla, the Dormammu TAC combo was:
TAC, ADDF, fly, j.LH, qcf.L, j.H, qcf.L, j.S
Then come Ultimate, either through frame data changes or just a new discovery, people started doing this:
TAC, j.MH, fly, j.LH, qcf.L, j.H, qcf.L, j.S
The problem with the Vanilla version is that ADDF doesn't let you complete the j.S at the end; Dormammu lands before the motion completes, and the combo drops.
So with Ultimate, people could not TAC up or down to Dormammu, and keep their same direction.
However, recently people discovered that there is a way to do it left/right as well:
TAC, ADDF (crosses under opponent), fly, j.LH, qcf.L, j.H, qcf.L, j.S
So now Dormammu can be TACed into in all three directions, build about half a bar of meter, and not lose your team's facing.
People forget, but this game is still young. Give it 2 more years, and everyone will be able to get full flight combos off of any TAC direction. To me, I don't care about the meter gain/loss. Dormammu can build a whole bar for me no matter what direction I go in with the right assists. A down TAC would give me 2 bars, but I'd rather be unpredictable.
Unless you are psychic, TACs are a 1/3 guess. It's like playing that 3 cup one ball game only you never get to see your opponent shuffle the cups in front of you. How can you possibly make a "read" on that?
Edit: Wow beaten badly, even with a video!
The video that Karst posted would be more effective if it was 3 cups instead of one and the poison was in two cups.
I prefer this analogy because the ending is more true: TACs are poison no matter what when you have to choose. ;-)