[gxti]

It's a sliding scale. For example, compare to the audiophile cult^Wcommunity. Most people don't know what good headphones are until they put on their first pair, but then they do there is no going back to the old, muddy $10 pair from Target. But then a select few take it to ridiculous levels and spend thousands of dollars on de-oxygenated, cryo-treated ethernet cables with electron flow arrows and ceramic ground plane spacing pylons. Don't let the fact that these weirdos exist (and the businesses willing to bilk them) fool you into thinking that most people couldn't tell the difference at the low end of the scale though, where each additional dollar goes the furthest. But most people never buy any headphones other than at the low end, so they don't realize that a modest increase in price can provide a substantial gain in quality.

The same thing is true about coffee -- supermarket beans are cheap and stale. Starbucks is substantially more expensive (mostly because it's sold by the cup) and significantly better (mostly because of copious quantities of sugar), but still not that great. Beans fresh from a local roaster sit on a critical point where more money creates a similar increase in quality. Below that point, a dollar produces many dollars in value. Above that point, a dollar produces less than a dollar in value. Putting a three-stage reverse osmosis water filter on your $4000 double-boiler, gold-plated, hand-operated espresso machine is way off into "pleasant waste of money" territory.

As for hot water specifically: hot water does in fact change taste because A) hot water dissolves things faster (like the insides of your pipes) and B) tanks tend to concentrate and add to the impurities. See http://everything2.com/title/Never+drink+or+cook+with+hot+ta... for some examples. Whether your hot water has perceptible taste differences depends on your hot water heater and the piping around it. Cold water is best not because it is cold but because it has less crud in it. Of course, the cold water may need filtering too if it doesn't taste relatively neutral out of the tap, like many municipal supplies.