I've done a few braids and I found it is much easier if you draw a line, stay-stitch, sew the border or sashing on, THEN cut the extra off after. Starch helps also.
My favorite way to do borders is Bonnie Hunter's way. No measuring tape involved and I have never had a wavy border since I started doing them this way.
http://www.quiltville.com/borderhints.shtml