When I have wavy edges, it's always for one of two reasons: the pieces at the edges of the blocks may have been cut on the bias, or I've had to ease in (or stretch out) the inner edge of the border which leaves the outer edge sort of fluttery. What I've done to fix it is steam the heck out of it and press it as flat as I can. And really I wouldn't worry myself about it - your quilt was done with love and no one but you probably notices that the edges wave.