Depending on what kind of texture/visual effect I want, I’ll use both solid and varigated threads, different numbers of strands, or different types of fiber, all in the same section.

Granted, I do most of my stitching on tightly woven fabric, which makes the placement of the knots easier. You could do it on needlepoint canvas quite easily, though. Either use thread with enough substance to keep it from slipping through the holes in the canvas, or place a piece of sheer fabric (organza, for example) over the canvas and stitch through both layers. The sheer fabric will keep the threads from slipping through the holes, and the knots will anchor the two layers together. You can get the knots together enough that the fabric/canvas doesn’t show through, even when using fairly small knots.

I LOVE making French knots, although I realize that I’m unusual in that regard.