Arcadia, 12 x 24 inches, oil on linen
So when do I do these things and when do I not? I don't have a rule set in stone and I tend to follow my whim about these matters, but I have found in general that toning the canvas and doing the grisaille is very helpful when I'm painting more tonally, or in a lower key.
The main reason for toning the canvas or doing the grisaille is to kill the extreme value (white) of the canvas so that when darker values are laid on top, we don't have distracting contrasts happening due to the bright white peeking through, in between brushstrokes. This is especially annoying when I'm painting dark trees, and seems like more work than necessary to have to knock them back afterwards.
Obviously, the darker the value of a given area, the greater the contrast between it and the white of the canvas. And if the focal area happens to be very dark, the distraction is amplified.
You can use this to your advantage, if your painting subject or style requires a lot of contrasty texture in that area. My paintings tend to be more quiet so I don't want a whole lot of that kind of activity which takes away from my statement.
Having a tone underneath your colors also helps to unify and harmonize various areas of the painting by creating a common denominator of color or tone that is sprinkled throughout the surface. Of course this would be moot if you don't let the tone show through.
But even if you cover up every square millimeter of the canvas with thick paint, the tone underneath can help to keep your harmony in check as you develop the surface.
I keep my grisaille very simple and loose. It's basically a map of value organization. I try to express the design in three values, maybe four. It's very important to keep your value structure simple if you want the design to "read". Too many values at this stage does nothing to organize the design - don't fool yourself (as I did in my early years) into thinking more information is better. The point is organization, not copying every value that you see. Even if you're painting very realistically, the subtle values must happen within the simple value structure. Keep squinting to make sure your initial design doesn't become fragmented as you develop your picture. It should still be there in the finished painting.
Sometimes I use a neutral monochromatic tone for the underpainting, sometimes a color is used to help create color harmony, and sometimes complementary colors are used to create a color contrast in an area - sort of a vibrating effect.
I don't usually do the complementary thing, as (again,) I'm looking for more of a quiet-ness in my painting and color vibrations create too much activity, but sometimes I use it in the foreground to bring that area come forward and accentuate the color saturation in that area. ...which is precisely what I did in the painting I'm showing above. The reddish brown tone under the green grass creates a rich color contrast in the foreground whereas in the background, I don't want that kind of activity because the atmospheric effect is more unifying.
So when do I not tone my canvas or do an underpainting? If I know I'm going to be painting in a more of a high-keyed, sunlight filled, impressionist approach, I'll just go right in with colors on a white surface. The white peeking through my colors brightens the entire painting. Sometimes the contrast and texture is part of the subject matter, like the shimmering sunlight reflecting off of water's surface. I usually don't like to use gimmicky effects, but if it's effective and doesn't look gimmicky, I think it's OK. If someone looks at my painting and says, "I love how you got that effect!", then it's too gimmicky. I want the viewer to say "what a beautiful painting", and not immediately focus on some technique.