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	$5.90	12.385%	$6.6307	$6.63	$6.65	$6.60
Purchase #2	$76.30	8.636%	$82.8893	$82.89	$82.90	$82.90
Purchase #3	$32.32	4.447%	$33.7573	$33.76	$33.75	$33.80
Purchase #4	$242.43	9.144%	$264.5978	$264.60	$264.60	$264.60
Purchase #5	$140.33	10.836%	$155.5362	$155.54	$155.55	$155.50
Purchase #6	$167.65	9.614%	$183.7679	$183.77	$183.75	$183.80
Purchase #7	$72.64	4.518%	$75.9219	$75.92	$75.90	$75.90
7 purchase totals =	$737.57		$803.1011	$803.11	$803.10	$803.10