I've been working on a Pi power cocktail cabinet and have found there are generally two ways to flip the games to 90 degrees for vertical mode. The first is to set the emulator to rotate the game to which ever rotation you want. This is nice if you plan to have non-vertical mode games show correctly. The difficulty with this is that the game's aspect ratio can get messed up. The second way to rotate is to set the Pi's boot setting to rotate the screen 90 degrees. This makes everything show in the vertical mode, including the frontend, command line, etc. The problem with this is that it degrades performance somewhat. I've done it on my Pi v3 and haven't noticed any issues so far. It is also nice in that it rotates the front end so that a player sitting at on end of the monitor will have the game list rotated for them. I use RetroPie with mame libretro as the emulator. Works well so far. Advanced Mame is also nice.