Of course now we'll get a whole discussion about due corde, 2.718281828459 corde, and finding a bit of as yet unused felt on the hammers, etc.... but my foot is not that
Well, there are two philosophies among the pianists, I think. One side says, a ppp should never be played with una corda, only if the composers prescribes una corda. A good pianists should be able to differenciate among p-pp-ppp without una corda. The other side says, it´s a legal matter to use the left pedal also for differenciation of the piano-grades. I´m just counting myself to that last group. But, of course, you need an instrument it is possible to realize that and last but not least that´s a pure matter of taste, isn´t it?