when we used to fit these in HIgh rise apartments they were fitted to the water inlet for each unit but in your case fitting it after the softner makes as much sense as anywhere. If you do this then your filters still get the benifit of full mains pressure to allow for losses and your pump gets a nice stable inlet pressure. Dont forget you may need to increase the pressure on your pump relief valve (if fitted) to allow for the reduced inlet pressure.
Pressure fluctuations on mains can be caused by several things more normal is a drop in pressure (caused by excessive usage other than at your place) than an increase. An increse may have been caused by a temporary change in water source by your water board or a pump or valve control malfunction or even a permanent change to the setup. Water boards use pressure limiters as well. A pressure limiting valve will solve this but like all valve long term need a little looking after in the long term (5-10 years) as if it sticks shut you will starve your pump of water :(