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	$36.36	6.648%	$38.7772	$38.78	$38.80	$38.80
Purchase #2	$162.91	8.874%	$177.3666	$177.37	$177.35	$177.40
Purchase #3	$53.79	10.443%	$59.4073	$59.41	$59.40	$59.40
Purchase #4	$48.26	12.746%	$54.4112	$54.41	$54.40	$54.40
Purchase #5	$234.98	6.822%	$251.0103	$251.01	$251.00	$251.00
Purchase #6	$55.27	7.494%	$59.4119	$59.41	$59.40	$59.40
Purchase #7	$123.50	13.412%	$140.0638	$140.06	$140.05	$140.10
7 purchase totals =	$715.07		$780.4483	$780.45	$780.40	$780.50