I really love shoulder-in -- or rather, shoulder-fore is where I'm at now. I don't so much do the movement, as use the movement to straighten and supple. I'm a sort of remedial-oriented rider, maybe because my first horse had so many problems. I'm not quite sure what anabel means by "on a line" but in any case, those exercises are far too advanced for us now!
I ride shoulder-in in 2 ways. The easiest is coming out of a circle, onto the center line, going down the middle. Where I ride is just a smoothish part of the pasture (yeah, lumpy) without a wall, so it's a little harder along the side. In that case, I do what I was told was the "better" way, which is to take the forehand over, bending around the leg. Although she'll turn on the outside rein, I find it difficult to keep her going along the side --- my pushing aid is too weak. Anyway, it's what I described: I end up using my leg further back, and it turns into a leg yield. I just brought this up because it's what came to mind when the OP wanted to talk "crookedness."
I like the idea of continuing in the circle at times. At least it would keep her inside leg more active.