How wavy? Maybe it's something that will even out in the wash? Do you see wavy lines when the quilt top is all put together?
When you are sewing, watch that your edges line up with something on the machine-- a line on the throat plate or the edge of the foot. Watch that space the whole time you sew and you should have a straight seam.
I don't starch anything. And I don't press excessively. I am happy with the seams I get.