It really depends on how much you want to watch it. I fished a tournament two summers ago where we trolled the face of a wing dam, and I turned my SI to only scan the wingdam side of the boat. I marked a pod of fish, dropped a waypoint on them, then went back and anchor locked above that waypoint. First cast 25″ eye, 2nd cast 20″ eye.
If you shrink your SI range, you get a clearer picture, I usually have mine set to about 50′ each way. I have been trolling many times and spotted pods of fish on the SI, marked waypoints and went back over them and caught fish. I have also used it to identify what areas fish are holding in. I’ll notice what appears to be a sand point, on Pepin lets say. As my boards go over said sand point (not visible to the naked eye as it’s all under water), boom a board goes back. Another pass, it happens again, I now have a pattern, and something to look for.
For long line trolling, I have marked fish on my SI, and have been able to estimate how far away from the boat they are and on which side. I’ve made the adjustment to troll over the areas I marked fish on, and have caught fish because of it.
To me, the return on investment for side imaging depends on how you’re going to use it. If you expect it to magically show you fish all the time, you’ll be sorely disappointed, but if you pay attention to it, and don’t try to look at too big of an area (like I said, I limit mine to 50′ each way), you’ll be able to put together the clues more easily.