To keep your topstitching straight, you can stick a stack of post-it notes where you want the edge of your project to be and follow along that. Lo and behold your stitching will be straight. Some feet have markings or places that you can eyeball to keep your stitches straight. My daughter always hated topstitching when I made her clothes because she said it was wiggly. Well, now I have many tricks that impress her when she sees my topstitching. As far as zig-zag vs straight, it is up to you. If you like it, that is all that matters. Also if you want your lining to "roll" to the inside of your project, press the seam toward the lining and then sew very close to that seam. Then when you turn the lining in the outside will roll toward the lining, making it easy to topstitch close to the edge.