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	$36.78	6.371%	$39.1233	$39.12	$39.10	$39.10
Purchase #2	$91.92	11.229%	$102.2417	$102.24	$102.25	$102.20
Purchase #3	$86.72	3.680%	$89.9113	$89.91	$89.90	$89.90
Purchase #4	$173.60	10.532%	$191.8836	$191.88	$191.90	$191.90
Purchase #5	$91.81	5.286%	$96.6631	$96.66	$96.65	$96.70
Purchase #6	$102.29	11.246%	$113.7935	$113.79	$113.80	$113.80
Purchase #7	$52.88	5.375%	$55.7223	$55.72	$55.70	$55.70
7 purchase totals =	$636.00		$689.3388	$689.32	$689.30	$689.30