Could someone please describe the exact difference between a "straight edge" razor versus a razor with a slight smiling curve? I'm mostly looking to figure out what distinguishes a razor that makes you use either the regular X stroke or the 45-degree X stroke.

I've never owned a brand new razor before, but it seems to me that all of the razors I've seen up close have at least a very small curve to them. I'm talking about tenths of a millimeter here. So how much of a smile does a razor have to have to figure out what stroke to use?

I know that the best answer to this is probably "try the different strokes out and see how they work," but I'm still not great at honing, and I'd like to eliminate as many variables as I can =)