The rules are pretty simple, and are determined by texture, water flow, and gravity.
Peeing in the shower when it's a smooth surface, with sufficient water force, that runs downhill is fine... the pee is going to go away from you at good speed, mix with plenty of clean water, and go down the drain.
Peeing in a tile floored shower, however, is a different story... some of the pee won't run downhill, it'll stay on top of the raised tiles where you risk stepping right into it. That's obviously not desired. You need to make sure that the water flow is sufficient to run over the top of the tiles, and not just through the grout. You also need to make sure it's sufficiently graded downhill toward the drain to take the pee away quickly. You need enough water to mix and dilute the pee, at least 10:1, and wash it away with force.
I'm surprised this is a question, it seems obvious.