Please learn to use fixed-point arithmetic, people... this is not a pet project anymore, it's a financial application that at the moment holds $181 million USD worth of people's savings.
I think you might need a lesson on how floating point works.
In short, results of rounding, calculations and comparisons may vary depending on compiler, optimizations, or architecture used.
Furthermore, there are other non-obvious semantics, such as negative zeros, infinities, NaNs, etc.
Double precision IEEE numbers have more than enough precision to represent any possible bitcoin amount.
As long as you represent things as integer nanocoins, and do only addition and subtraction, none of the things mentioned are going to have any effect.
BASIC used double precision floats for everything for decades, and it worked just fine for integer calculations, within a reasonable range.
If you multiply and divide, and want things like banker's rounding, then floating point can be a bit problematical, but for tallying transactions, it works fine.