Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
Case	Tag   Price	Sales   Tax %	Raw Total   Incl. Sales Tax	Rounded   to Penny	Rounded   to Nickel	Rounded   to Dime
Purchase #1	$240.93	13.484%	$273.417	$273.42	$273.40	$273.40
Purchase #2	$181.72	13.645%	$206.5157	$206.52	$206.50	$206.50
Purchase #3	$139.71	3.322%	$144.3512	$144.35	$144.35	$144.40
Purchase #4	$36.45	6.148%	$38.6909	$38.69	$38.70	$38.70
Purchase #5	$112.92	5.154%	$118.7399	$118.74	$118.75	$118.70
Purchase #6	$249.56	9.564%	$273.4279	$273.43	$273.45	$273.40
Purchase #7	$247.73	12.281%	$278.1537	$278.15	$278.15	$278.20
7 purchase totals =	$1,209.02		$1,333.2963	$1,333.30	$1,333.30	$1,333.30