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	$51.69	5.391%	$54.4766	$54.48	$54.50	$54.50
Purchase #2	$37.41	12.711%	$42.1652	$42.17	$42.15	$42.20
Purchase #3	$134.77	8.773%	$146.5934	$146.59	$146.60	$146.60
Purchase #4	$28.41	5.385%	$29.9399	$29.94	$29.95	$29.90
Purchase #5	$95.72	9.819%	$105.1187	$105.12	$105.10	$105.10
Purchase #6	$58.75	12.462%	$66.0714	$66.07	$66.05	$66.10
Purchase #7	$187.38	3.473%	$193.8877	$193.89	$193.90	$193.90
7 purchase totals =	$594.13		$638.2529	$638.26	$638.25	$638.30