All calculations with currency values and amounts should be computed and stored using integer arithmetic only, as opposed to floating point, which can lead to round off errors.
BitCoin is divisible to the 8th decimal point and could be expanded in the future. Integer-only transactions won't work.
Actually, that is precisely why floating point calculations don't work. Floating point calculations introduce round off error and are unable to store certain numbers. Integer calculations can handle any decimal point and can store any number, given enough bits.
Integer arithmetic with numbers that have decimal places are handled by performing all internal calculations with numbers multiplied by, say 100 million. No loss of precision ever occurs, unlike with floating point numbers. To print the number to the user simply requires inserting a dot at the appropriate place.