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	$6.77	13.223%	$7.6652	$7.67	$7.65	$7.70
Purchase #2	$23.41	11.330%	$26.0624	$26.06	$26.05	$26.10
Purchase #3	$114.28	12.778%	$128.8827	$128.88	$128.90	$128.90
Purchase #4	$110.59	9.662%	$121.2752	$121.28	$121.30	$121.30
Purchase #5	$41.33	9.740%	$45.3555	$45.36	$45.35	$45.40
Purchase #6	$86.38	5.113%	$90.7966	$90.80	$90.80	$90.80
Purchase #7	$106.90	4.393%	$111.5961	$111.60	$111.60	$111.60
7 purchase totals =	$489.66		$531.6337	$531.65	$531.65	$531.80