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	$87.36	3.189%	$90.1459	$90.15	$90.15	$90.10
Purchase #2	$228.35	10.261%	$251.781	$251.78	$251.80	$251.80
Purchase #3	$168.71	5.279%	$177.6162	$177.62	$177.60	$177.60
Purchase #4	$145.49	13.333%	$164.8882	$164.89	$164.90	$164.90
Purchase #5	$19.46	10.857%	$21.5728	$21.57	$21.55	$21.60
Purchase #6	$49.46	8.452%	$53.6404	$53.64	$53.65	$53.60
Purchase #7	$28.78	5.136%	$30.2581	$30.26	$30.25	$30.30
7 purchase totals =	$727.61		$789.9026	$789.91	$789.90	$789.90