I discovered after a lot of frustration (and some great help from Nic) that one of my conditional currency fields wasn't working, and the cause was that I hadn't set the default value to "0.00". As a result, any attempt to compare numbers (greater than / less than values) on a record where one of the values was set to "blank" instead of "0", it did not compute correctly.
Without knowing other problems this might create, I think it would be really helpful to people if the default value of currency fields was set to "0.00" as the default and not require the user to put it in.
Until then, I at least learned my lesson, and perhaps it will help others.
Benjamin