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	$39.44	8.638%	$42.8468	$42.85	$42.85	$42.80
Purchase #2	$37.97	7.380%	$40.7722	$40.77	$40.75	$40.80
Purchase #3	$152.50	11.445%	$169.9536	$169.95	$169.95	$170.00
Purchase #4	$36.71	6.135%	$38.9622	$38.96	$38.95	$39.00
Purchase #5	$100.46	9.569%	$110.073	$110.07	$110.05	$110.10
Purchase #6	$98.28	7.253%	$105.4082	$105.41	$105.40	$105.40
Purchase #7	$80.41	3.106%	$82.9075	$82.91	$82.90	$82.90
7 purchase totals =	$545.77		$590.9235	$590.92	$590.85	$591.00