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	$36.62	7.347%	$39.3105	$39.31	$39.30	$39.30
Purchase #2	$143.44	11.503%	$159.9399	$159.94	$159.95	$159.90
Purchase #3	$50.80	13.813%	$57.817	$57.82	$57.80	$57.80
Purchase #4	$231.33	12.318%	$259.8252	$259.83	$259.85	$259.80
Purchase #5	$148.48	9.324%	$162.3243	$162.32	$162.30	$162.30
Purchase #6	$66.75	8.503%	$72.4258	$72.43	$72.45	$72.40
Purchase #7	$246.62	3.564%	$255.4095	$255.41	$255.40	$255.40
7 purchase totals =	$924.04		$1,007.0522	$1,007.06	$1,007.05	$1,006.90