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	$88.38	12.754%	$99.652	$99.65	$99.65	$99.70
Purchase #2	$124.42	11.762%	$139.0543	$139.05	$139.05	$139.10
Purchase #3	$71.86	11.307%	$79.9852	$79.99	$80.00	$80.00
Purchase #4	$90.48	10.262%	$99.7651	$99.77	$99.75	$99.80
Purchase #5	$50.54	7.723%	$54.4432	$54.44	$54.45	$54.40
Purchase #6	$54.67	3.495%	$56.5807	$56.58	$56.60	$56.60
Purchase #7	$240.29	9.568%	$263.2809	$263.28	$263.30	$263.30
7 purchase totals =	$720.64		$792.7614	$792.76	$792.80	$792.90