The shock extenders will rip off your shock mounts, eventually. Too much leverage.
And as someone said, it's handy if you take our your rear springs to do the grinding. But, IIRC, it may not be necessary.
Of course, I'd recommend doing the AEV / Nth Degree shock shifter kit. It does wonders for the handling/ride of the rear end of the TJ. I did my own by trying to reverse design the Nth kit. . . after doing that I FULLY recommend just getting the Nth kit. It'll save you boatloads of time. This is more involved than just cutting the spring perch but it'll give you a nice improvement in the ride of your TJ. No more back-end wiggle going over bumps and the rig feels more planted.
The SS mod is a step between running shocks in the stock location and doing the full frame-cut, long-travel shock mod.