Be careful - as you undo the threaded portion, the bottom race will loosen up, and depending on how much grease/gunge there is and how good the bearing cage is, the bearings might just fall out all over the floor! If the balls in the bottom part are OK, you probably don't even need to take it all apart - you can just undo the top bolt a bit, drop the fork down a cm or 2, give everything an inspection, clean, and grease, and then push it back up again (if you put the fork ends on the floor then you can do the same with the top race at this point).
So long as you are able to correctly tighten it afterwards, inspecting and re-greasing a threaded headset can be done remarkably quickly.