For me it depends on the quilt. I do a lot of scrappy quilts, and for scrappy quilts I usually throw in at least one scrappy-pieced border (if I have borders at all). Those I usually just make extra long and then chop down to size.
If it's a non-scrappy or a very controlled scrappy, I measure out and either adjust the block size to fit, or I'll divide the border into halves or quarters or thirds and use specifically-sized "key" blocks in between each segment to make it all work out.
Mostly I just sorta fly by the seat of my pants and figure it out as I go, though. My quilts are pretty casual so that works for me.
MOST of my quilts have no borders at all.