Yes, that's the point I was making.

    "I don't remember the last time I owed π dollars to someone"

Well, you did say "you can store any number"  :-)

But even if we do limit the calculations to strictly financial ones, it's very easy to demonstrate a case where you'll get round-off.

Assume you have invested $1000, and your average annual interest rate is exactly 4%.  According to the The Rule of 72, you can expect your money to double in approximately (72 / 4) = 18 years.

The problem is, this isn't exact.  The exact period after which your money will double is given by the formula ln 2 / ln 1.04, which is approximately 17.67298769 years.  But my point is, no module is going to give you 100% accuracy for that, because ln 2 and ln 1.04, as well as their quotient, are all transcendental numbers, which means you can't represent them 100% accurately.  Which is all I was trying to get across.