Force on edge is important, not strop taughtness
In my experience it is the force on the edge of the razor from it's weight against strop's surface that makes a difference to how the edge is shaped/polished, not whether the strop is kept taught or allowed to sag. Pesonally I don't worry about it and allow the strop to sag an inch or more in the middle. I just be careful to keep the spine just barely touching the strop and maintain a moderate to light weight on the edge.
Keeping the strop fairly taught is certainly sound advice, though. It's kinda' cool how there is a lot of room for personal style with these fascinating shaving tools.