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	$18.82	9.258%	$20.5624	$20.56	$20.55	$20.60
Purchase #2	$228.67	8.690%	$248.5414	$248.54	$248.55	$248.50
Purchase #3	$220.91	9.821%	$242.6056	$242.61	$242.60	$242.60
Purchase #4	$144.74	12.470%	$162.7891	$162.79	$162.80	$162.80
Purchase #5	$238.29	13.293%	$269.9659	$269.97	$269.95	$270.00
Purchase #6	$89.77	11.874%	$100.4293	$100.43	$100.45	$100.40
Purchase #7	$79.55	4.229%	$82.9142	$82.91	$82.90	$82.90
7 purchase totals =	$1,020.75		$1,127.8079	$1,127.81	$1,127.80	$1,127.80