Rubber Seat Washers in Shutoff Valve

When our house was built, the plumber installed 3/4" copper runs to the bathroom rough-in in the basement. At the end of each run, he put a shutoff valve with rubber seat washers; the valves also have the small drain cock on the side containing a rubber seal.

When I sweat the outlet pipe to these valves, can I do it with the water on and the valves closed, or will the heat melt the rubber washers?

You can solder as is without shutting off the water, but do not overheat. Remove the bleeder or small drain cock before soldering