I change it up. Sometimes I like to tone the canvas with an orangey or reddish undercoat. A lot of times I'll add an underpainting after I've blocked something in...only with the the magic of digital. More often than not, this underpainting will be a photo of an actual painting...which provides the illusion of additional 'tooth' for the brushstrokes, and provides an extra layer of texture complexity when finished, as little bits show through the final strokes. Sometimes I'll combine a photo of a painting with an orange undercoat, and sometimes I'll leave the canvas white. Other times I'll remove the underpainting layer after I've painted on a layer over the top for a while, which gives me some unexpected results with the stark white showing through. I like to experiment a lot.