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	$247.78	12.751%	$279.3744	$279.37	$279.35	$279.40
Purchase #2	$101.81	13.440%	$115.4933	$115.49	$115.50	$115.50
Purchase #3	$91.53	7.514%	$98.4076	$98.41	$98.40	$98.40
Purchase #4	$78.62	3.571%	$81.4275	$81.43	$81.45	$81.40
Purchase #5	$66.99	8.810%	$72.8918	$72.89	$72.90	$72.90
Purchase #6	$172.80	6.677%	$184.3379	$184.34	$184.35	$184.30
Purchase #7	$53.53	3.216%	$55.2515	$55.25	$55.25	$55.30
7 purchase totals =	$813.06		$887.1840	$887.18	$887.20	$887.20