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	$19.66	3.319%	$20.3125	$20.31	$20.30	$20.30
Purchase #2	$29.88	6.196%	$31.7314	$31.73	$31.75	$31.70
Purchase #3	$148.76	11.199%	$165.4196	$165.42	$165.40	$165.40
Purchase #4	$24.79	4.777%	$25.9742	$25.97	$25.95	$26.00
Purchase #5	$79.69	13.477%	$90.4298	$90.43	$90.45	$90.40
Purchase #6	$23.71	13.492%	$26.909	$26.91	$26.90	$26.90
Purchase #7	$45.71	11.565%	$50.9964	$51.00	$51.00	$51.00
7 purchase totals =	$372.20		$411.7729	$411.77	$411.75	$411.70