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	$26.71	12.480%	$30.0434	$30.04	$30.05	$30.00
Purchase #2	$194.83	12.546%	$219.2734	$219.27	$219.25	$219.30
Purchase #3	$158.86	5.373%	$167.3955	$167.40	$167.40	$167.40
Purchase #4	$129.38	3.770%	$134.2576	$134.26	$134.25	$134.30
Purchase #5	$48.69	10.682%	$53.8911	$53.89	$53.90	$53.90
Purchase #6	$10.38	10.731%	$11.4939	$11.49	$11.50	$11.50
Purchase #7	$197.58	13.137%	$223.5361	$223.54	$223.55	$223.50
7 purchase totals =	$766.43		$839.8910	$839.89	$839.90	$839.90