Do professional online players always prefer to play on a chessboard oriented with the Black pieces on the top or with their own pieces on the bottom? Is there a consensus on this, and has there ever been a formal or informal poll?
Since over the board chess is played with each player's pieces starting on the side closest to them, the natural board orientation is with your own pieces on the bottom. This is the default orientation when a game starts on all the major chess-playing sites and is how all the top players like Nakamura, Carlsen, etc. play when they stream on Twitch. It is not common to play in the other orientation unless it for a specific reason, like as a challenge.
There are occasions where flipping the board can be helpful still, presumably even at the top level, such as when trying to remember opening theory that you play as one color but not the other. However, most grandmasters could visualize the flipped board in their head just as easily as flipping the orientation.
As a ~2000-rated player on Lichess, I occasionally find it helpful to analyze positions from the opponent's orientation before my move, especially when I'm making a choice between attacking moves. Having lost so many games when the opponent made a strong move into my position that I had not considered, I am usually better able to judge whether a move will be crushing or defendable from the defending orientation. However, this is also not so relevant for professionals since they are skilled at analyzing the board from any orientation. Part of the reason they are strong players is because their ability to see the best moves on the board is not directional.