I am starting to put the borders on my butterfly quilt. Getting to this point means a) you're about finished and b) a whole other set of possible problems just opened up. There's the design selection thing (plain? pieced? narrow? wide?); the corner construction thing (lapped? mitered?); the edge and corner finish thing (squared? rounded? scalloped?); and of course the accuracy thing (i.e., please don't let me distort this quilt top out of recognition as a parallel-sided geometric form!).
I like striped borders and usually use the stripes crosswise. That means stretchy borders and mitered corners, two possible areas fraught with difficulties. Once I get the striped mitered border attached, I usually use lapped corners on the outer border, if there is one. I know that if you have several borders, it may be faster to sew them together and apply as one mitered piece but the lapped corners seem to add stability to the quilt shape.
I also like to cut the borders length of grain if possible to add more stability, but if I get cheap I'll just cut them cross-grain to save fabric. I like the look of a fairly wide border in the focus fabric (from 3/4 of a block to a full block wide), to tie all the colors of the pieced top together. This works better on larger quilts.
But best of all, adding the first border strips lets you get a glimpse of the finished design and gives you a final chance to make design changes. I, however, once famously did not pay attention at this point and had to hang the finished top up for photography before it hit me that the border was wrong, wrong, wrong.
The picture is just a teaser: you'll see the completed quilt top later. Two more border segments and my butterfly nap quilt top will be completed, and I wouldn't change a thing. But -- I'm piecing the backing, so I am far from done!