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	$24.50	13.335%	$27.7671	$27.77	$27.75	$27.80
Purchase #2	$100.50	8.695%	$109.2385	$109.24	$109.25	$109.20
Purchase #3	$149.31	5.118%	$156.9517	$156.95	$156.95	$157.00
Purchase #4	$73.40	6.661%	$78.2892	$78.29	$78.30	$78.30
Purchase #5	$68.51	7.580%	$73.7031	$73.70	$73.70	$73.70
Purchase #6	$62.91	13.628%	$71.4834	$71.48	$71.50	$71.50
Purchase #7	$68.37	6.410%	$72.7525	$72.75	$72.75	$72.80
7 purchase totals =	$547.50		$590.1855	$590.18	$590.20	$590.30