Does the installation of a Reverse Osmosis Filter in the water line do the job of water softening?

And do I care if I brew with soft water or not?

The water around here is extremely hard.

(from a newbie)

Check this out: Jim Schulman's Insanely Long Water FAQ

The short answer is: don't use RO water in your espresso machine.

There are several reasons:

1. If your machine has level probes to autofill the boiler(s) or guard against low water level, they probably won't work with RO. The probes need mineral ions in the water that can conduct electricity. The amount of mineral content doesn't have to be high, but it can't be zero.

2. In some cases, RO water can be slightly acidic. That's not good for your machine.

3. The water must have a certain minimum mineral content in order to properly extract the coffee. This is discussed at length in Jim's water FAQ.

That said, very hard water will cause scaling in your machine, which is also bad. You can regularly descale, but that has been known to produce undesirable results, such as scale particles flaking off and clogging small orifices, acid corrosion, etc.

The first thing to do is get a water hardness testing kit so you can determine if your water requires softening. This one from API will do, or you can get more accurate and more expensive kits from Hach.

If the hardness level is high enough to cause scaling problems, you can use a softening system to eliminate the hardness without removing all the mineral content. Here are a couple from my dealer, Chris Coffee:

http://www.chriscoffee.com/products/home/plumbing/softenersystem

http://www.chriscoffee.com/products/home/plumbing/commercialsofteningsys

I use the commercial system. There are other technologies available that produce RO water and let you mix in tap water to raise the mineral content enough for proper extraction, but due to concerns about acidic water, some manufacturers will void the warranty on your machine if you use them.