Read mounting instructions....
before you buy. Most better quality units will have an athwartship adjustment, or include a template for building a mounting block. Not sure what Vic means that it doesn't matter. It matters to me... I use it to detect windshifts and tacking points. It's difficult to steer to an exact card point because the boat yaws and twists as it moves on it's course. The yawing and twisting constantly changes the direction the lubber lines are reading, but the boat itself is still moving in the general course direction. A magnetic compass does not need, or consider, boat movement in it's measurement. It simply tells you the direction of magnetic north. It can do this at a standstill. The GPS, on the other hand, needs to have movement to determine direction. Therefore, it is desirable to have both devices on your boat. At the helm, you can become mesmerized by staring at the compass, trying to stay on a specific point. What you try to do is steer a basically straight course using external reference points, if possible, regularly checking the compass for verification. Observing the boat's wake is a good way to determine if you're steering a straight course.