Lethn:There's an even easier way of checking for that awful wonkiness if you're using photoshop and you don't even have to use image flip if you want to be super careful about it, create a layer then draw a verticle line with the marquee, because it's a computer it will draw it perfectly straight every time so you don't have to flip any time you want to check for straightness.
Sorry Lethn, but that's not a good idea. Flipping canvas is the best method because you see the issues right away. It is not a good idea to use guides to try to get an image right the first time.
Yian 2012