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	$16.41	12.817%	$18.5133	$18.51	$18.50	$18.50
Purchase #2	$100.54	11.310%	$111.9111	$111.91	$111.90	$111.90
Purchase #3	$32.95	13.472%	$37.389	$37.39	$37.40	$37.40
Purchase #4	$110.38	8.791%	$120.0835	$120.08	$120.10	$120.10
Purchase #5	$153.52	11.106%	$170.5699	$170.57	$170.55	$170.60
Purchase #6	$49.74	5.741%	$52.5956	$52.60	$52.60	$52.60
Purchase #7	$86.62	4.454%	$90.4781	$90.48	$90.50	$90.50
7 purchase totals =	$550.16		$601.5405	$601.54	$601.55	$601.60