Re: stuck valve
It depends on your intentions. If you're trying to turn off the valve to work on the fixture, don't bother, as you'll likely just cause it to leak. It will be much easier to turn off the water main coming into the house than to mess with the anglestop.
If you are wanting to replace the anglestop with a new one, can you describe the current valve? Is it a compression style that has a nut screwed to it, securing it to the pipe, or is the "nut" part of the valve, indicating that it's a threaded valve. You might even be able to see threads on the length of pipe coming out of the wall if it is a threaded valve.
Also, what is the diameter of the pipe coming out of the wall? If it's copper and give-or-take 1/2" in diameter, then you've likely got a compression valve. Use two wrenches placed on the valve and nut with the handles forming a "V" that you can squeeze together to break the nut loose. If the pipe is give or take 3/4" in diameter, it is a threaded valve. If there is enough stub sticking out of the wall, grab it with vise grips, then set a wrench on the valve to form a "V" to break the joint loose.
Remember, before doing anything, turn the water off at the main.
Good luck and report back
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!