Any of these curves has the potential to stretch if you are not careful. Convex curves do not have to be clipped so much as the concave ones. There are some nifty video clips on You Tube and also on the Quilt Show that show some fool proof methods of temporarily gluing the pieces together and then sewing. I recommend you watch those tutorials. You could also try to place your pattern so that the convex curve was mostly on the warp grain of the fabric in order to eliminate stretch, but anything cut on the bias will be stretchy. Even the weft of the fabric is more stretchy than the warp. All the best with the next project.
P.S. I have a sewing foot called 'the curve master' which is ideal for sewing curved seams. There is a video on You Tube about it as well.