I think that this is a simple install. The supply pipe is steel. The exit connection will be using fleible 3/8 in hose. This is for a sink faucet. My worry is loosining the steel pipe connection at the other end while taking off the old ball joint. Does any on have any tips /methods regarding the removal of the old ball joint without causing any leaks.

Always use two wrenches.

Adjustable or regular when flats are available.

The marring method:
Use a pipe wrench as one of the wrenches.

The non-marring method.
Use a strap wrench as one of the wrenches.
Goggle search of images with title "strap wrench".
strap wrench - Google Image Search (http://images.google.com/images?client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hl=en&q=strap+wrench&btnG=Search+Images&gbv=2)

I think that this is a simple install. The supply pipe is steel. The exit connection will be using fleible 3/8 in hose. This is for a sink faucet. My worry is loosining the steel pipe connection at the other end while taking off the old ball joint. Does any on have any tips /methods regarding the removal of the old ball joint without causing any leaks.
I've read your question and am still a little confused.

My worry is loosining the steel pipe connection at the other What other end are you referring to? What's "the old ball joint"?By steel pipe do you mean galvanized? Is this the pipe that comes out of the wall? What, exactly do you wish to accomplish? Changing out the flexible supply? Remove and replace a ball jont stop from the wall stubout? If all you wish to do is replace the angle stop then shut the water off at the house stop and place one pipe wrench on the wall stubout so the wrench is on the right hand side so it doesn't turn and the other on the angle stop. Using one wrench as a backstop vyou may now back the old angle stop back out counterclockwise. When replacing the angle stop don't forget to use Teflon Tape on the threads. Good luck, Tom

I think he is concerned that when he attempts to remove the old cutoff valve by screwing it off the galvanized pipe, he might unscrew the pipe itself from the fitting inside the wall and wants to avoid doing that.