When I got back into quilting in 2000 I read a lot of books to learn how folks approached quilt design. Several of the books I read talked about borders and the information was interesting (though I do not pretend that I have adhered to the advice at all times). Here are some of the things that I picked up:
-- what is the purpose of the border -- is it to simply enlarge the size of the top, is it to draw the eye out of the quilt or into the center, or is it to stop the pattern so you can maybe do a patterned border as well
-- If the quilt has pieced blocks, no individual border should be as large as the block
-- if there are multiple borders, it is most pleasing to have the narrowest toward the center
-- In planning for multiple borders, and odd numbers are better than even
-- better no border than the wrong border
-- for medallion quilts it is most pleasing to alternate pieced and solid borders so the eye has a place to rest -- alternating the size of the borders is good too.
-- some designers tell you that if the blocks of the quilt are large, the total size of all borders should not exceed the size of the blocks.
Since there are no quilt police, I use these concepts as guidelines then do my own thing.