Craft Commerce: Custom Rounding (for taxes and discounts)
The rounding you are talking about is for cash. The rounding is to nearest 5c because it is likely the smallest denomination in circulation.

Online, with electronic transactions, the base will always be whole cents (or localised equivalent.) Credit Card and electronic purchases can always transaction in these minor units.

Craft Commerce doesn't have the ability to round to cash-based values (likely to be used in POS), but you are welcome to make a feature request here: http://feedback.craftcms.com/