If I got it right, while wo is being used normally for questions about a position of a state, wohin is being used normally for questions about a final position of a movement.
Various clear examples include:
Wo liegt das Buch? / Wohin soll ich das Buch legen?
Wo stehen die Blumen? / Wohin sollen wir die Blumen stellen?
Wo sitzt Hans? / Wohin soll ich das Baby setzen?
(I am not completely sure about the last example)
However, at least in some of the cases of reflexive verbs, I couldn't find a clear pattern. I usually distinguish ich sitze from ich setze mich as thinking of the latter as an action I am doing on myself (just like doing it "on" Hans, the baby). So I would expect the following:
Wohin soll ich mich setzen?
But I do see a lot of these:
Wo soll ich mich setzen?
In this page I even saw both forms in the same "conversation", so I'm quite confused. I can just guess that both forms are alright, but if that's the case, is it only with setzen? Is it with reflexive verbs in general?