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	$135.96	8.492%	$147.5057	$147.51	$147.50	$147.50
Purchase #2	$241.33	4.210%	$251.49	$251.49	$251.50	$251.50
Purchase #3	$80.96	11.442%	$90.2234	$90.22	$90.20	$90.20
Purchase #4	$167.38	10.515%	$184.98	$184.98	$185.00	$185.00
Purchase #5	$147.78	5.477%	$155.8739	$155.87	$155.85	$155.90
Purchase #6	$56.72	7.745%	$61.113	$61.11	$61.10	$61.10
Purchase #7	$70.28	12.385%	$78.9842	$78.98	$79.00	$79.00
7 purchase totals =	$900.41		$970.1702	$970.16	$970.15	$970.20