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Never had to replace them myself - the 12" I bought had the job already done. From reports, 8" seemed to be OK, 12" seemed to definitely need stronger springs to prevent slight sag. 10" in the middle - so maybe.
One approach is to pull the mirror further down towards the bottom of the tube so the springs are under increased pressure and should be less prone to any sag. When I bought my second-hand scope, the previous owner had one collimation screw pulled up tight and adjusted primary collimation with the other two - hence keeping the springs well compressed. I've since given them all a bit more slack so I can play with all three.
It's just, if you are changing the knobs, the springs should be a simple change at the same time.
I reckon the secondary screws are more important to avoid dropping that screwdriver or Allen wrench onto the primary - Ouch!
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