I heard that you should just steer whilst the blades are in the water to reduce drag and maintain the set. Does that mean I only move the strings when the blades are in, or do I return them to the straight position during the recovery? The latter doesn’t seem like it would turn the boat much.
During the recovery yes, unless you’re going around a really sharp curve, you would go back to having the rudder straight and the strings even. In my opinion the whole “steering while the blades are in the water” thing only truly applies to when you’re racing, especially during sprint races, because you don’t want to be steering much to begin with and, obviously, you don’t want to slow it down by creating drag from the rudder. During practice it’s not as big of a deal but the same idea still applies.
Like you said, going back and forth between having the rudder turned and having it straight won’t turn the boat as much as just holding it in the direction you want to go, but the “con” of that is that it’ll affect the set of the boat (more or less depending on the skill level of the rowers you’re coxing). You should only hold it to the side like that though if you’re trying to get around a big curve or a sharp turn. In cases like this, just tell your rowers that you’re steering to port/starboard (you should do this regardless), and have them adjust their handle heights as necessary. (Don’t forget to tell the when you’re off the rudder too so they can readjust their hands.)
So, to answer your question, when you’re going straight or going around very slight curves, you should make small adjustments as necessary and go back to having the strings straight on the recovery. I usually steer for a stroke, take a stroke or two off, and then make another adjustment if I have to. If you’re going around a larger turn just hold the rudder to the side you’re trying to go and have the rowers adjust their handle heights until you’re finished steering.