A couple of the above posts refer to a 'drip leg' to collect water and other debris in the line. If the drip leg did collect water, wouldn't it freeze and possibly split the pipe over time causing a serious and dangerous leak?
The drip leg should be at least 3/4"x 4-6" long this will hold a good amount of water. If it fills up with water in less than a year you have a larger problem. The cap should be removed and checked at least once a year.
When I put in a drip leg, I'll add a drain valve to get rid of accumulated water. I don't know what others do.
I agree with what Skip said. I would not put a valve on the leg. Just use a pipe cap. A valve would be to easy for some one to open or accidently hit and crack it open.