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	$18.59	9.552%	$20.3657	$20.37	$20.35	$20.40
Purchase #2	$117.67	12.498%	$132.3764	$132.38	$132.40	$132.40
Purchase #3	$72.71	10.374%	$80.2529	$80.25	$80.25	$80.30
Purchase #4	$95.99	7.593%	$103.2785	$103.28	$103.30	$103.30
Purchase #5	$100.61	11.445%	$112.1248	$112.12	$112.10	$112.10
Purchase #6	$71.45	13.292%	$80.9471	$80.95	$80.95	$80.90
Purchase #7	$223.57	7.418%	$240.1544	$240.15	$240.15	$240.20
7 purchase totals =	$700.59		$769.4998	$769.50	$769.50	$769.60