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	$19.98	5.574%	$21.0937	$21.09	$21.10	$21.10
Purchase #2	$119.75	6.735%	$127.8152	$127.82	$127.80	$127.80
Purchase #3	$46.64	8.839%	$50.7625	$50.76	$50.75	$50.80
Purchase #4	$31.37	9.144%	$34.2385	$34.24	$34.25	$34.20
Purchase #5	$237.85	13.701%	$270.4378	$270.44	$270.45	$270.40
Purchase #6	$114.52	11.851%	$128.0918	$128.09	$128.10	$128.10
Purchase #7	$133.36	4.767%	$139.7173	$139.72	$139.70	$139.70
7 purchase totals =	$703.47		$772.1568	$772.16	$772.15	$772.10