Marking seams on an edge should mirror automatically if x/y/z symmetry is on

When you mark seams on selected edges, it won't mirror to the other side if symmetry is on. It should automatically do so if you have symmetry on, it doesn't make sense to force the user to mirror their selection to do this automatically, this goes for other operations like smooth vertices and loop cuts too. There should be an option at least to make operations mirror their selection beforehand if you have symmetry on already.

I've had too many cases of the model accidentally becoming asymmetrical because I forgot to mirror selection before doing an operation and I have to revert a lot of work.