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	$37.47	12.189%	$42.0372	$42.04	$42.05	$42.00
Purchase #2	$83.63	6.371%	$88.9581	$88.96	$88.95	$89.00
Purchase #3	$56.75	9.321%	$62.0397	$62.04	$62.05	$62.00
Purchase #4	$106.88	4.651%	$111.851	$111.85	$111.85	$111.90
Purchase #5	$183.26	11.479%	$204.2964	$204.30	$204.30	$204.30
Purchase #6	$32.34	5.562%	$34.1388	$34.14	$34.15	$34.10
Purchase #7	$93.45	5.771%	$98.843	$98.84	$98.85	$98.80
7 purchase totals =	$593.78		$642.1642	$642.17	$642.20	$642.10