You might be able to press them straight. Make sure you're not pulling the strip as you're sewing - I've done that. If you have a walking foot, you might try that to see if it helps prevent the bowing.
Also, if you're sewing more than 2 strips together, alternate the ends where you start. In other words, when you sew strip A to strip B, start at the north end, then when you sew strip C, start at the south end.