The sound on these puppies comes out from everywhere, lol...
You've got a lot of choices, but I'd classify them into four groups: (1) out the top, (2) from the back, (3) under the front, and (4) more ambient from farther back.
As discussed already, a lot of this is personal preference, and another big part is how the thing is positioned (against wall or not, etc.). Also, if you are dealing with other instruments at the same time, bleed from them on the piano mics will play a role. Another thing to consider is the piano itself -- for example, if the piano is bright, you might not want to go out the top. Personally, I often go with split omnis near the top and I haven't had too much trouble with mechanical noise. A dynamic under the front can be good for bleed issues (if not inside with the lid down), except possibly clugy foot pedals can cause mech noise. Moving back and going more ambient can work, but I've found that doesn't work nearly as well as it does with a grand.
Bottom line, is there is no real right/wrong way with an upright. I t depends a lot on preference, the piano itself, the config in the room, bleed issues, what kind of mics you have, etc. If you can, experiment. If not, go with your gut. That's my two cents at least.