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	$36.53	9.221%	$39.8984	$39.90	$39.90	$39.90
Purchase #2	$139.91	9.170%	$152.7397	$152.74	$152.75	$152.70
Purchase #3	$109.71	12.402%	$123.3162	$123.32	$123.30	$123.30
Purchase #4	$145.85	10.592%	$161.2984	$161.30	$161.30	$161.30
Purchase #5	$136.44	6.352%	$145.1067	$145.11	$145.10	$145.10
Purchase #6	$249.94	9.464%	$273.5943	$273.59	$273.60	$273.60
Purchase #7	$65.75	10.594%	$72.7156	$72.72	$72.70	$72.70
7 purchase totals =	$884.13		$968.6693	$968.68	$968.65	$968.60