It is your wall hanging, so, you make the rules! What is the most pleasant to your eye? Audition a couple different looks on the opposite sides. Leave it up and whenever you enter the room, which side grabs your eye? This is the side you will always prefer. This works for color and for width. There are no quilt police (other than at shows!), so you make it the way that makes you smile and feel good.

I realize I am a day(or 2) late. I attended a quilting school and this is what I learned. The size of the block determines the size of your border. Thus 8 inch block would look better with a 4 inch border. Sashings should be 2 inches or less but more than an inch. Do I go by this? No! I just happen to like 4 or 4.5 inch borders. The teacher was a retired garment industry employee from NY.

it often depends on how much fabric i have left...... :-) but i also consider the outer border to be the 'frame' of the quilt so i don't want it to dominate. mine (for a full or queen size--haven't done a king yet) usually wind up being about 4". for a lap quilt, i generally use 3"

Never heard of making borders to match width of blocks. Sounds like 2" for inner boarder and 4-6 for outer. Don't think I would go over 6" on the outer.

I usually do the same as MarleneC & plan for it when shopping. If pulling from my stash, that may or may not work.