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	$37.57	12.401%	$42.2291	$42.23	$42.25	$42.20
Purchase #2	$139.44	3.776%	$144.7053	$144.71	$144.70	$144.70
Purchase #3	$63.76	13.717%	$72.506	$72.51	$72.50	$72.50
Purchase #4	$158.80	4.356%	$165.7173	$165.72	$165.70	$165.70
Purchase #5	$157.57	9.515%	$172.5628	$172.56	$172.55	$172.60
Purchase #6	$15.33	13.250%	$17.3612	$17.36	$17.35	$17.40
Purchase #7	$25.74	12.499%	$28.9572	$28.96	$28.95	$29.00
7 purchase totals =	$598.21		$644.0389	$644.05	$644.00	$644.10