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	$24.40	6.196%	$25.9118	$25.91	$25.90	$25.90
Purchase #2	$127.82	12.112%	$143.3016	$143.30	$143.30	$143.30
Purchase #3	$174.94	6.627%	$186.5333	$186.53	$186.55	$186.50
Purchase #4	$190.56	8.683%	$207.1063	$207.11	$207.10	$207.10
Purchase #5	$127.67	12.519%	$143.653	$143.65	$143.65	$143.70
Purchase #6	$114.52	9.112%	$124.9551	$124.96	$124.95	$125.00
Purchase #7	$81.39	10.392%	$89.848	$89.85	$89.85	$89.80
7 purchase totals =	$841.30		$921.3091	$921.31	$921.30	$921.30