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	$13.67	11.503%	$15.2425	$15.24	$15.25	$15.20
Purchase #2	$204.52	11.728%	$228.5061	$228.51	$228.50	$228.50
Purchase #3	$186.39	7.256%	$199.9145	$199.91	$199.90	$199.90
Purchase #4	$103.28	11.352%	$115.0043	$115.00	$115.00	$115.00
Purchase #5	$25.37	11.801%	$28.3639	$28.36	$28.35	$28.40
Purchase #6	$174.33	9.479%	$190.8547	$190.85	$190.85	$190.90
Purchase #7	$48.82	7.587%	$52.524	$52.52	$52.50	$52.50
7 purchase totals =	$756.38		$830.4100	$830.39	$830.35	$830.40