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	$36.75	9.626%	$40.2876	$40.29	$40.30	$40.30
Purchase #2	$39.92	6.771%	$42.623	$42.62	$42.60	$42.60
Purchase #3	$77.40	6.391%	$82.3466	$82.35	$82.35	$82.30
Purchase #4	$89.88	7.104%	$96.2651	$96.27	$96.25	$96.30
Purchase #5	$194.78	10.473%	$215.1793	$215.18	$215.20	$215.20
Purchase #6	$113.81	10.656%	$125.9376	$125.94	$125.95	$125.90
Purchase #7	$36.80	5.293%	$38.7478	$38.75	$38.75	$38.70
7 purchase totals =	$589.34		$641.3870	$641.40	$641.40	$641.30