What RPM to shift at?

All 5.0L's are differnet depending on mods, trans, gear, tune etc.

My personal 5.0L 91 Mustang 5spd car...mods in sig. I shift on the street at 5,900RPM. My car has a sweet spot there. It really moves out well. Others may say, that is too high, but you can feel the power is still there.

I have read 5,600-5,700 is the highest a stock form 5.0L should be shifted, but you are gonna get many answers on this one.
 
True, after 5,200 sometimes my car falls off a little bit....I know I said I shift at about 5,900, and I do, but I think it's when my car is hot and the outside temp is hot is when it falls off a little. You can hear it just for a split second.