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	$18.63	13.412%	$21.1287	$21.13	$21.15	$21.10
Purchase #2	$90.88	10.376%	$100.3097	$100.31	$100.30	$100.30
Purchase #3	$105.48	10.351%	$116.3982	$116.40	$116.40	$116.40
Purchase #4	$86.83	5.392%	$91.5119	$91.51	$91.50	$91.50
Purchase #5	$111.91	7.119%	$119.8769	$119.88	$119.90	$119.90
Purchase #6	$135.47	4.834%	$142.0186	$142.02	$142.00	$142.00
Purchase #7	$201.36	13.543%	$228.6302	$228.63	$228.65	$228.60
7 purchase totals =	$750.56		$819.8742	$819.88	$819.90	$819.80