There is no way of doing this globally except altering the display money function, but yes, if you edit eCart, you will have to re edit the format number function.
Format number is used to format the display of the number, it will force the number of decimals you tell it to display. This comes into play with round round numbers like 11. if you tell format number to use 2 decimals, it form at 11 as "11.00", whereas round(11, 2) will only return "11". Format number will perform the rounding function though.