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	$250.32	5.798%	$264.8336	$264.83	$264.85	$264.80
Purchase #2	$154.59	10.400%	$170.6674	$170.67	$170.65	$170.70
Purchase #3	$160.32	7.250%	$171.9432	$171.94	$171.95	$171.90
Purchase #4	$150.74	11.267%	$167.7239	$167.72	$167.70	$167.70
Purchase #5	$133.83	9.148%	$146.0728	$146.07	$146.05	$146.10
Purchase #6	$28.54	6.261%	$30.3269	$30.33	$30.35	$30.30
Purchase #7	$38.50	9.359%	$42.1032	$42.10	$42.10	$42.10
7 purchase totals =	$916.84		$993.6710	$993.66	$993.65	$993.60