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	$38.32	10.590%	$42.3781	$42.38	$42.40	$42.40
Purchase #2	$122.50	8.516%	$132.9321	$132.93	$132.95	$132.90
Purchase #3	$98.34	3.625%	$101.9048	$101.90	$101.90	$101.90
Purchase #4	$199.51	6.399%	$212.2766	$212.28	$212.30	$212.30
Purchase #5	$26.34	13.232%	$29.8253	$29.83	$29.85	$29.80
Purchase #6	$96.51	13.201%	$109.2503	$109.25	$109.25	$109.30
Purchase #7	$37.91	7.481%	$40.746	$40.75	$40.75	$40.70
7 purchase totals =	$619.43		$669.3132	$669.32	$669.40	$669.30