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	$51.82	4.785%	$54.2996	$54.30	$54.30	$54.30
Purchase #2	$60.53	10.541%	$66.9105	$66.91	$66.90	$66.90
Purchase #3	$91.42	11.821%	$102.2268	$102.23	$102.25	$102.20
Purchase #4	$229.57	3.357%	$237.2767	$237.28	$237.30	$237.30
Purchase #5	$228.74	9.190%	$249.7612	$249.76	$249.75	$249.80
Purchase #6	$34.31	13.202%	$38.8396	$38.84	$38.85	$38.80
Purchase #7	$80.76	12.616%	$90.9487	$90.95	$90.95	$90.90
7 purchase totals =	$777.15		$840.2631	$840.27	$840.30	$840.20