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	$10.92	12.752%	$12.3125	$12.31	$12.30	$12.30
Purchase #2	$117.44	10.123%	$129.3285	$129.33	$129.35	$129.30
Purchase #3	$148.88	13.533%	$169.0279	$169.03	$169.05	$169.00
Purchase #4	$136.41	5.606%	$144.0571	$144.06	$144.05	$144.10
Purchase #5	$93.55	11.339%	$104.1576	$104.16	$104.15	$104.20
Purchase #6	$189.88	3.642%	$196.7954	$196.80	$196.80	$196.80
Purchase #7	$92.41	8.280%	$100.0615	$100.06	$100.05	$100.10
7 purchase totals =	$789.49		$855.7405	$855.75	$855.75	$855.80