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	$35.33	11.853%	$39.5177	$39.52	$39.50	$39.50
Purchase #2	$70.54	11.749%	$78.8277	$78.83	$78.85	$78.80
Purchase #3	$79.91	10.692%	$88.454	$88.45	$88.45	$88.50
Purchase #4	$144.75	12.142%	$162.3255	$162.33	$162.35	$162.30
Purchase #5	$170.83	3.347%	$176.5477	$176.55	$176.55	$176.50
Purchase #6	$33.87	4.326%	$35.3352	$35.34	$35.35	$35.30
Purchase #7	$54.37	5.613%	$57.4218	$57.42	$57.40	$57.40
7 purchase totals =	$589.60		$638.4296	$638.44	$638.45	$638.30