Throughout the history of art, the way that the edges of two dimensional works are treated has varied wildly.
Often, edges are used to further the illusion of realistic painting. The scene depicted runs right up to the borders, as if the frame is enclosing a window rather than a canvas. As if the view is real, not simply a painting.
Early Christian painting held a different attitude. Images were centered within the surface, similar to low-relief carvings, which they were modelled after.
In my own abstract work, I like to be aware of the parameters. Sometimes I use pattern and color to draw the eye to the edge, then back in, to the center. Other times, I place an object in the center, and frame it in. Sometimes the border is negated by lines zipping off the surface, as if the image continues off the picture plane; other times the border is emphasized. In every case, the treatment of edges is important to the overall feeling.