Ignoring existing reality with things like banks taking inflation into account for long-term 30-year mortgages, what is the ideal inflation rate?
It seems like the answer should be 0%, but why?
And what exactly does it mean to have 0%?
Where economics is concerned, there is more nonsense vomited forth from all quarters than any sane person should think possible.
That said, the true measure of economic health as far as your question is concerned is "purchasing power". Globally speaking (i.e. the overall purchasing power of any given individual, all else equal) individual purchasing power should remain steady perpetually, barring some extraordinary circumstance. If $1 buys a pound of some "average commodity" on day one, in a healthy economy it will purchase the same amount at some arbitrarily chosen date in the future. This is, of course, a very simplistic model, but it serves to illustrate the basic principle regarding the fundamental nature of sound money systems. There are several complicating factors that may alter this at the microscopic level and, perhaps one a longer time scale, the macroscopic.
Example: advances in widget manufacturing technologies drastically reduces costs, thereby causing widget prices to drop (possibly very significantly) over a relatively short time frame (could be long, too). In such a case, there is a shift in purchasing power per unit of currency for a given commodity, constituting a microscopic economic shift in overall purchasing power.
If, however, a disruptive technology becomes available that significantly reduces the manufacturing costs of nearly all products and commodities, then we would likely witness an increase in overall purchasing power constituting a macroscopic change in the economy. The effects of such systemic changes are not entirely predictable in the current system, but in a sound monetary environment, such technological advances should result in a net INCREASE in the average wealth of all people because their purchasing power has gone up, not down.
Therefore, sound money systems stand only to increase wealth rather than to ever decrease them. But even with sound money there is not guarantee that some circumstance might arise whereby average wealth decreases because purchasing power is diminished. Natural disasters and war come to mind as the most obvious and extreme examples of macroscopic effects. At the microscopic level, some shortage in a natural resource may result in a loss of purchasing power of that given commodity. For example, if drought or some pestilence strikes global corn crops and corn becomes scarce, the price will go up and the effective purchasing power of a unit of money will be diminished relative to corn. But these sorts of losses are usually transitory, but not always.
An example of this would be the case of wootz, a type of steel that became to be known as "damascus steel" or "watered steel". Wootz was legendary in that it made some of the best swords in the world and it was believed for a very long time that its qualities were imparted by virtue of the secret process for smelting the iron. As it turned out, the reason for wootz's superior qualities was the naturally occurring vanadium, which imparts toughness. When the ore vein ran out, the smelters closed shop because the other veins did not have the unknown additive and the steel they produced thereafter did not have the same qualities. Because of this, the purchasing power of any money gradually reduced with the availability of wootz. Once exhausted, no amount of money could purchase it because there was no longer any to be had. This microscopic loss of purchasing power was "permanent". It happens, but is relatively very uncommon.
As we can see, things are not so simple where the relationship between money and economies are concerned. Nevertheless there is a basic principle of monetary action that in part governs the way economies function and imparts to them a strong measure of stability in general, all else equal. This is a strong reason for having a properly sound system of money; it enhances prosperity all around, whereas unsound and easily manipulated systems of currency can be used to strip away general prosperity while shielding some favored group. But perhaps I digress.
Anyhow, all else equal, average purchasing power should remain constant over time. There should be no inflation stemming from a devaluation of the currency. With technological advances, average purchasing power should actually go up per unit money. Were this is not the case in general, I would tend to be fearful that some sort of broad issue of sustainability had arisen, which of course would be cause for great concern. That or war.
Does any of this make sense to you?