> Most people don't realize that the IEEE-754 single precision floating point represent real numbers with 9 decimal digits (or 23 binary digits). The double, on the other hand, represents the real numbers with 17 decimal digits.

No, they don't. They merely can be converted back to decimal with those numbers of significant digits without loss of information.

That is important because (a) if this matters, you have to make sure you actually control the number of significant digits when converting to decimal, or you might end up with a different decimal, and (b) the operations that you do on the floats do not reliably behave as if there was the supposedly represented decimal number stored in them.

Now, sure, you can use floats for currency, if you know what you are doing, but the point of the warning against it is that you have to know what you are doing, and chances are you don't, or if you do, then you know where you can ignore it anyway.

(That is, unless you mean nothing more than that you can encode the information contained in an n-digit decimal in a float/double--which of course is true, but not particular to floating point numbers, as any state with a certain number of bits can, of course, encode any information of no more than that many bits, somehow.)

World GDP is around 87 trillion dollars.

    $ ruby -e 'pp 87e12 + 0.01'
    87000000000000.02

If you're certain that your software will never handle national-economy-scale or hyperinflationary use cases, then sure, you may be able to get away with 64-bit floats, but I think "no need to worry" is overstating your case. Please do worry about precision until you've proven you don't need to.

Fun, tangential anecdote:

I worked with a CSV containing, among other things, phone numbers. A coworker called and complained that the phone numbers were all wrong. He'd edited the thing in MS Excel, which promptly converted the phone numbers to floating point with a loss in precision. When he saved it, those new numbers were happily written back to the disk.