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	$37.73	12.130%	$42.3066	$42.31	$42.30	$42.30
Purchase #2	$234.71	10.615%	$259.6245	$259.62	$259.60	$259.60
Purchase #3	$22.43	10.269%	$24.7333	$24.73	$24.75	$24.70
Purchase #4	$159.92	7.393%	$171.7429	$171.74	$171.75	$171.70
Purchase #5	$5.66	6.411%	$6.0229	$6.02	$6.00	$6.00
Purchase #6	$37.31	4.555%	$39.0095	$39.01	$39.00	$39.00
Purchase #7	$37.46	13.391%	$42.4763	$42.48	$42.50	$42.50
7 purchase totals =	$535.22		$585.9160	$585.91	$585.90	$585.80