Well, if you have a defined storage goal, and you're quite certain that goal won't change... Just 9 months ago, my pool consisted of 3 x 2 TB disks in RAIDZ1, though it was getting full. I added 3 x 3 TB disks (another RAIDZ1) about the time I discovered SickBeard and CouchPotato. Now that's full. I just ordered 6 x 4 TB disks, and I'm glad I have a 12-bay chassis to install them in.
Part of "defined goal" as I use it implies that this isn't your first rodeo :) and you have taken into account the forseeable but not exactly countable-right-now needs (the "known unknowns").
The unknown unknowns are a different story. I consider data to be the most expensive thing to both get and keep. The hardware that keeps it is to a degree trivial.
And I do still have this goliath Norco that was too big for my needs, and a couple of Antec P-series chassis to expand into if need be. :) Moving a motherboard, power supply and disks into a bigger chassis takes about an afternoon.
On the other hand, I also know the rate at which jmy data storage needs have grown in the last ten years, and I'm not much into storing video or wrestling other gigantic files around. I put in about four times the storage that I will reasonably need for the near future. The scheme (it's a scheme, not a plan, if you don't know *exactly* how it will get done) is to let the continued increases in drive density let me upgrade as I go. My first NAS was Solaris based, about the time that Oracle ra... er, bought Sun and took all of its stuff internal. I got on to FreeNAS looking for an alternate ZFS vehicle. That NAS had 12 disk drives at a lofty 750GB each for about 10 TB in RAIDZ2.
The most recent has let me have 12TB in seven 3TB drives. I've not come close to 80% with either of these or the intermediates.
So I get away with having piddly little data storage, and some OK-so-far guesses about what I'll need.