In my nefarious travels through virtual space I have been summarizing what other artists are saying about the use and benefits of edges in paintings.
Here is a distillation:
One of the keys to good edges is variety. Edges clarify and unify a composition and affect how the painting is read. Edges have a role in defining shapes at their boundaries. Too many distinct edges can stiffen and isolate/disconnect the subject from the background. A few sharp edges focuses the viewer on the subject and anchors the center of interest. Our eyes are comfortable with this normal view. Edges can be soft or blended with a smooth transition, lending depth and harmony. Edges can be lost (+shape is the same value as the –shape). Edges can be broken, with an abrupt path. Edges can be inquiring, acknowledging and emphasizing small variations on the perimeter. Edges can have a close value (perhaps a warm and cool version of the same hue), for example, showing distance effects on two parts of a same colored wall. Shadow or soft edges can ground objects and avoid a floating appearance.
Yes. Color check every step. You are training your eye and brain 'til you don't need to check any more.
Find the complement colour on a color wheel and adjust the intensity to match your palette value. Gradually add this complement value to the palette value. This will mute but not changethe palette value.
Technique Starting in a corner of the painting, dip a lint free cloth into the turpentine and gently rub the surface of the painting. The varnish should come off onto the cloth - remember to keep a close eye on the cloth and if you see colour coming off then stop. Working in small squares, make your way across the surface of the painting and try to keep using a fresh piece of cloth, as this will help lift the varnish rather than simply spreading it about.