It all depends on the hardness of your water. Some cities have very hard water and others are not so hard. So the results of using a shower filter would depend on the hardness of the water you have in your town. A shower filter is only $40 and that's a cheap price to pay for extending the life of your system. But if it's impractical then you can do without it. Mineral water is fine to rinse your hair. And maybe the water in your town is naturally soft and you can just use that.
Regards,
John