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	$122.67	10.446%	$135.4841	$135.48	$135.50	$135.50
Purchase #2	$237.34	12.211%	$266.3216	$266.32	$266.30	$266.30
Purchase #3	$82.27	8.634%	$89.3732	$89.37	$89.35	$89.40
Purchase #4	$55.78	10.207%	$61.4735	$61.47	$61.45	$61.50
Purchase #5	$31.66	5.303%	$33.3389	$33.34	$33.35	$33.30
Purchase #6	$16.85	9.777%	$18.4974	$18.50	$18.50	$18.50
Purchase #7	$237.89	11.790%	$265.9372	$265.94	$265.95	$265.90
7 purchase totals =	$784.46		$870.4259	$870.42	$870.40	$870.40