if they're wavy after you've sewn them together and pressed, it may be that the two different fabrics react differently to the heat of the iron - especially if you're using steam and/or spray starch.
i take one each of the different strips i'm going to use over to the ironing board and test their reaction to pressing before i sew them together. if the fibres contract or expand when the spray starch hits them, i know to press all the strips of that particular fabric before i sew them with them.
it's a pain and slows me down, but it takes less time than ripiting the strips apart to fix them.