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	$122.76	11.702%	$137.1254	$137.13	$137.15	$137.10
Purchase #2	$164.47	12.612%	$185.213	$185.21	$185.20	$185.20
Purchase #3	$68.73	5.103%	$72.2373	$72.24	$72.25	$72.20
Purchase #4	$47.25	9.419%	$51.7005	$51.70	$51.70	$51.70
Purchase #5	$224.65	12.790%	$253.3827	$253.38	$253.40	$253.40
Purchase #6	$52.77	7.602%	$56.7816	$56.78	$56.80	$56.80
Purchase #7	$78.64	5.375%	$82.8669	$82.87	$82.85	$82.90
7 purchase totals =	$759.27		$839.3074	$839.31	$839.35	$839.30