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	$84.32	6.170%	$89.5225	$89.52	$89.50	$89.50
Purchase #2	$156.27	8.249%	$169.1607	$169.16	$169.15	$169.20
Purchase #3	$240.91	9.772%	$264.4517	$264.45	$264.45	$264.50
Purchase #4	$47.47	10.487%	$52.4482	$52.45	$52.45	$52.40
Purchase #5	$48.59	10.366%	$53.6268	$53.63	$53.65	$53.60
Purchase #6	$81.72	10.112%	$89.9835	$89.98	$90.00	$90.00
Purchase #7	$43.71	13.572%	$49.6423	$49.64	$49.65	$49.60
7 purchase totals =	$702.99		$768.8357	$768.83	$768.85	$768.80