No, there's no reason NOT to solder the supply valves on to the water supply piping to a washing machine.

I have 3 washing machines in my building and all of the water supply shut off valves are soldered into place. And, I have ball valves up stream of those water shut off valves so that I can service the shut off valves without shutting off the water to all three washing machines, and those ball valves are soldered in too.