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	$43.27	12.760%	$48.7913	$48.79	$48.80	$48.80
Purchase #2	$50.45	8.637%	$54.8074	$54.81	$54.80	$54.80
Purchase #3	$182.33	5.234%	$191.8732	$191.87	$191.85	$191.90
Purchase #4	$103.70	6.513%	$110.454	$110.45	$110.45	$110.50
Purchase #5	$31.88	9.583%	$34.9351	$34.94	$34.95	$34.90
Purchase #6	$94.90	4.578%	$99.2445	$99.24	$99.25	$99.20
Purchase #7	$88.87	13.502%	$100.8692	$100.87	$100.85	$100.90
7 purchase totals =	$595.40		$640.9747	$640.97	$640.95	$641.00