Someone asked me yesterday if I sketch out my paintings first?
My answer was "no" because we were talking about oil paintings. I do not sketch out my oil paintings ahead of time but sketch as I go, drawing one or two shapes, blocking them in, drawing the next couple of shapes, blocking them in, etc. until the entire composition is complete. Of course, with oil paint, you can change things as you go.
For my botanicals I do several studies first and sketch the final detailed composition on the watercolor paper. For general watercolors, I do a combination of the two styles, sketching out large parts of the composition with my watercolor brush and a neutral color before beginning to block in areas.