I usually just but/nestle my blocks together - usually they turn out okay enough.
However, I always measure my quilts in a couple of places to get an average length before I put the border on. (Never on the edge)
Sometimes my edges ruffle or stretch a bit from handling or ironing or whatever.
Then if my blocks are averaging 12 inches finished (for example) I will make a mark 13 inches from the end, and then 12 inches for however many blocks i have on a side. Then I pin at each intersection. I sew with the border on top (if it's a strip - then it's easier to ease those slightly larger pieces in - if necessary)