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	$249.69	12.291%	$280.3794	$280.38	$280.40	$280.40
Purchase #2	$181.96	4.227%	$189.6514	$189.65	$189.65	$189.70
Purchase #3	$236.85	13.815%	$269.5708	$269.57	$269.55	$269.60
Purchase #4	$111.28	10.817%	$123.3172	$123.32	$123.30	$123.30
Purchase #5	$181.58	8.440%	$196.9054	$196.91	$196.90	$196.90
Purchase #6	$88.48	6.443%	$94.1808	$94.18	$94.20	$94.20
Purchase #7	$163.31	9.736%	$179.2099	$179.21	$179.20	$179.20
7 purchase totals =	$1,213.15		$1,333.2149	$1,333.22	$1,333.20	$1,333.30