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	$37.73	11.570%	$42.0954	$42.10	$42.10	$42.10
Purchase #2	$171.37	11.702%	$191.4237	$191.42	$191.40	$191.40
Purchase #3	$58.28	6.277%	$61.9382	$61.94	$61.95	$61.90
Purchase #4	$144.43	11.622%	$161.2157	$161.22	$161.20	$161.20
Purchase #5	$217.49	9.654%	$238.4865	$238.49	$238.50	$238.50
Purchase #6	$90.96	6.323%	$96.7114	$96.71	$96.70	$96.70
Purchase #7	$88.35	13.185%	$99.9989	$100.00	$100.00	$100.00
7 purchase totals =	$808.61		$891.8698	$891.88	$891.85	$891.80