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	$242.55	13.743%	$275.8836	$275.88	$275.90	$275.90
Purchase #2	$39.31	10.422%	$43.4069	$43.41	$43.40	$43.40
Purchase #3	$229.40	12.416%	$257.8823	$257.88	$257.90	$257.90
Purchase #4	$36.50	10.495%	$40.3307	$40.33	$40.35	$40.30
Purchase #5	$67.81	4.229%	$70.6777	$70.68	$70.70	$70.70
Purchase #6	$90.87	10.245%	$100.1796	$100.18	$100.20	$100.20
Purchase #7	$28.43	11.631%	$31.7367	$31.74	$31.75	$31.70
7 purchase totals =	$734.87		$820.0975	$820.10	$820.20	$820.10