Depending on how many numbers you process, inefficiency might not matter.
You could probably use arbitrary precision rational numbers (eg as implemented in Python https://docs.python.org/3/library/fractions.html) for all your accounting needs, and it would still be fast enough.
If you want to do financial pricing of lots of instruments, performance is important.
If you want to do the accounting for one company (that is not trading in millions of financial instruments but, say, is building and selling widgets), you have far fewer numbers on your books.
Decimal floating-point gives you the range to cover everything but is generally very inefficient to process.
What you usually want is fixed-point decimal, with the caveat that the scaling factor will be different per asset.