I developed an issue where my whole foot started hurting on longer rides with my mtb shoes. I ended up moving the cleat back about 4mm and using the back hole to allow the metal plate that it attaches to, to be more forward on the bottom of my foot...worked for me.

I just threw them on wherever originally and didn't think twice about it (about 2 years ago) and never had an issue. There is so much float in my shoes that I don't think it really mattered. I think I started having issues when I took training more seriously this year and started pushing pretty decent sustained wattage, in all honesty.