Ideally there should be enough room for asymmetry in detailing that not every single prop has to be perfectly mirrored on both sides anyway. Look at Granary, with its different styles of architecture, or Double Cross where a pile of crates on one side is an upturned truck full of sand on the other. To say nothing of how much flexibility you have once you get outside the field of play.
And I do feel like that's something we've have lost sight of. Almost all the successful community maps have had sides that are perfect copypastes of each other with the colors changed. Whether it's from the mapmaker getting tired of detailing at some point, or something competitive players demand, or maybe a little of both.
Point is, if you don't like the idea of a red tank on BLU's side, you shouldn't feel obligated to put one there. You can put something completely different there instead! As long as it's similar enough to not mess with the layout.