I have never actually given much thought to why cross stitch patterns say to start in the middle, I have just done it. Now that I do think about it am not sure what the reason is.

Since most of my cross stitching is done on packs where I get the pattern, the cotton and the aida, I haven't had a problem with running out of space. I think it was my Mother who taught me the trick of folding the aida double and then running my nail along the crease (and then repeating in the other direction) to find the middle of the material.