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	$145.64	6.561%	$155.1954	$155.20	$155.20	$155.20
Purchase #2	$99.85	10.643%	$110.477	$110.48	$110.50	$110.50
Purchase #3	$78.71	10.326%	$86.8376	$86.84	$86.85	$86.80
Purchase #4	$18.63	9.561%	$20.4112	$20.41	$20.40	$20.40
Purchase #5	$54.26	10.243%	$59.8179	$59.82	$59.80	$59.80
Purchase #6	$58.83	7.348%	$63.1528	$63.15	$63.15	$63.20
Purchase #7	$134.99	12.281%	$151.5681	$151.57	$151.55	$151.60
7 purchase totals =	$590.91		$647.4600	$647.47	$647.45	$647.50