I would think it depends on the mileage of your car. After time your stock ones will wear out then I would replace with stock or comparable spring rate. I wouldn't even mess with em unless you feel you suspension needs to be revived. Cutting a coil on a somewhat fresh stock spring could be a good way to go if you wanna get rid of the "Mustang Rally Edition" look.
H&R Super Sport-
I have FRPP "B" springs in my profile picture below. I went the economy route, found them dirt cheap at the time.
You can look at the lowering spring picture data base on sn95forums to compare options, FYI
__________________ '97 3.8L V6 Pacific Green 1996SVTCobraWhite