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	$15.82	13.782%	$18.0003	$18.00	$18.00	$18.00
Purchase #2	$197.84	10.849%	$219.3037	$219.30	$219.30	$219.30
Purchase #3	$210.41	7.559%	$226.3149	$226.31	$226.30	$226.30
Purchase #4	$219.67	10.682%	$243.1351	$243.14	$243.15	$243.10
Purchase #5	$37.33	13.538%	$42.3837	$42.38	$42.40	$42.40
Purchase #6	$213.51	4.133%	$222.3344	$222.33	$222.35	$222.30
Purchase #7	$94.34	11.618%	$105.3004	$105.30	$105.30	$105.30
7 purchase totals =	$988.92		$1,076.7725	$1,076.76	$1,076.80	$1,076.70