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	$19.71	7.236%	$21.1362	$21.14	$21.15	$21.10
Purchase #2	$164.89	11.494%	$183.8425	$183.84	$183.85	$183.80
Purchase #3	$139.90	12.713%	$157.6855	$157.69	$157.70	$157.70
Purchase #4	$114.58	11.499%	$127.7556	$127.76	$127.75	$127.80
Purchase #5	$54.84	12.803%	$61.8612	$61.86	$61.85	$61.90
Purchase #6	$93.33	4.514%	$97.5429	$97.54	$97.55	$97.50
Purchase #7	$169.67	9.390%	$185.602	$185.60	$185.60	$185.60
7 purchase totals =	$756.92		$835.4259	$835.43	$835.45	$835.40