A sketch is a sketch is a sketch . . . .
worthwhile or a waste of valuable time that I could be spending painting?
Well, it's like pointing the car in the right direction and just hoping you get where you want to go. Your sketch is the MAP: a way to master the view, instead of letting it master you.
Here are some simple tips for sronger value sketches:
1. Use a viewer to limit and "trap" the view so that you know exactly what will be included in the frame; both positive, as well as negative shapes. I use an empty cardboard slide holder. (remember slides?)
2. Simplify, simplify, simplify.
Start with a sketch (or Notan) - limit detail by squinting so you see less and can combine areas of detail into larger areas of value that are similar.
3. Stick with 3 values.
Remember that step-by-step process a few posts back?
The lights which are the white of the paper, the darkest darks which are as dark as you can make them with the pencil, and the middle tones in between. Simplifying the sketch to just 3 values will communicate your ideas more powerfully - and translate more easily to color.
4. Make value changes for a better composition.
Once the sketch is done, you have a map for yourself but it may become apparent that not every route is a good one. Think about rearranging value areas or connecting them to improve the composition or to better lead the eye through the painting.