If the piano is tuned properly, by a trained professional, then all the notes will be equally "in tune" and all the 5th will be identical. It goes without saying that this doesn't apply if you have a bad piano tuner. ;D
But there is no such thing as one single equal temperament. There are many different permutations.
12-TET is favoured by some tuners, but the results can be quite unsuccessful with thirds (see above)
There is some more information about 12-TET, and the reasons & places it goes wrong on pianos, here on WikiPedia. Some of this stuff is specific to pianos and their mechanisms. There is a detailed listing of the mathematically 'correct' frequencies of all 88 notes of a concert grand, but in reality - explanation given there too - these are not always the best choice. So when you ask for 12-TET, in 95% of cases you're still not getting it, but the tuner's corrected version of it instead.31-TET is used by many professional tuners these days. It tempers the fifths, in order to get the thirds in closer.
There is also 41-TET, which is a decent compromise, but there are still some ropey fifths. The tuners at the Moscow Conservatoire use 41-TET, unless given other instructions, for example. 53-TET is the closest usual tuning to strict Pythagorean, but it drives people bonkers. It's used in Turkish music, though.
Some performers of atonal music go for 72-TET, which is a correcter "mathematical" squeezing. But I don't know any tuners who will put this tuning onto an instrument that is intended for general use - and in fact many would baulk at it entirely.
It's worth remembering that 'just temperament' is not the same as 'equal temperament'.
It's also worth remembering that the words 'temper' and 'tamper' have the same root :)
