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	$186.64	12.350%	$209.69	$209.69	$209.70	$209.70
Purchase #2	$91.94	12.764%	$103.6752	$103.68	$103.70	$103.70
Purchase #3	$133.93	7.164%	$143.5247	$143.52	$143.50	$143.50
Purchase #4	$235.41	6.126%	$249.8312	$249.83	$249.85	$249.80
Purchase #5	$7.89	7.605%	$8.49	$8.49	$8.50	$8.50
Purchase #6	$31.61	6.413%	$33.6371	$33.64	$33.65	$33.60
Purchase #7	$82.88	8.606%	$90.0127	$90.01	$90.00	$90.00
7 purchase totals =	$770.30		$838.8609	$838.86	$838.90	$838.80