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	$249.25	12.515%	$280.4436	$280.44	$280.45	$280.40
Purchase #2	$31.87	4.457%	$33.2904	$33.29	$33.30	$33.30
Purchase #3	$130.85	10.593%	$144.7109	$144.71	$144.70	$144.70
Purchase #4	$143.58	7.451%	$154.2781	$154.28	$154.30	$154.30
Purchase #5	$36.50	10.649%	$40.3869	$40.39	$40.40	$40.40
Purchase #6	$45.28	6.247%	$48.1086	$48.11	$48.10	$48.10
Purchase #7	$151.47	3.263%	$156.4125	$156.41	$156.40	$156.40
7 purchase totals =	$788.80		$857.6310	$857.63	$857.65	$857.60