Most healing spells have a 40 yard range, so you should be able to break follow and happily heal away (unless your warrior gets feared I guess), I'd try to keep your shaman fairly close though as you might run the risk of tagging in extra adds with it and it will be easier to make sure you're in line of sight with your warrior if you can quickly see where your shaman is (I know I have more trouble keeping track of two toons locations if they are further apart).