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	$88.45	12.799%	$99.7707	$99.77	$99.75	$99.80
Purchase #2	$216.45	7.685%	$233.0842	$233.08	$233.10	$233.10
Purchase #3	$159.53	12.808%	$179.9626	$179.96	$179.95	$180.00
Purchase #4	$59.89	8.356%	$64.8944	$64.89	$64.90	$64.90
Purchase #5	$194.28	4.560%	$203.1392	$203.14	$203.15	$203.10
Purchase #6	$114.69	5.260%	$120.7227	$120.72	$120.70	$120.70
Purchase #7	$82.69	7.512%	$88.9017	$88.90	$88.90	$88.90
7 purchase totals =	$915.98		$990.4755	$990.46	$990.45	$990.50