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	$18.83	13.109%	$21.2984	$21.30	$21.30	$21.30
Purchase #2	$155.64	10.711%	$172.3106	$172.31	$172.30	$172.30
Purchase #3	$102.29	12.814%	$115.3974	$115.40	$115.40	$115.40
Purchase #4	$148.39	11.767%	$165.8511	$165.85	$165.85	$165.90
Purchase #5	$44.97	6.860%	$48.0549	$48.05	$48.05	$48.10
Purchase #6	$77.75	11.348%	$86.5731	$86.57	$86.55	$86.60
Purchase #7	$164.46	11.307%	$183.0555	$183.06	$183.05	$183.10
7 purchase totals =	$712.33		$792.5410	$792.54	$792.50	$792.70