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	$39.99	5.145%	$42.0475	$42.05	$42.05	$42.00
Purchase #2	$166.55	7.661%	$179.3094	$179.31	$179.30	$179.30
Purchase #3	$32.46	4.471%	$33.9113	$33.91	$33.90	$33.90
Purchase #4	$31.99	4.544%	$33.4436	$33.44	$33.45	$33.40
Purchase #5	$100.79	5.585%	$106.4191	$106.42	$106.40	$106.40
Purchase #6	$118.99	4.471%	$124.31	$124.31	$124.30	$124.30
Purchase #7	$19.78	4.745%	$20.7186	$20.72	$20.70	$20.70
7 purchase totals =	$510.55		$540.1595	$540.16	$540.10	$540.00