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	$19.99	3.390%	$20.6677	$20.67	$20.65	$20.70
Purchase #2	$101.63	10.515%	$112.3164	$112.32	$112.30	$112.30
Purchase #3	$81.92	4.450%	$85.5654	$85.57	$85.55	$85.60
Purchase #4	$231.30	9.728%	$253.8009	$253.80	$253.80	$253.80
Purchase #5	$43.90	8.468%	$47.6175	$47.62	$47.60	$47.60
Purchase #6	$35.86	13.427%	$40.6749	$40.67	$40.65	$40.70
Purchase #7	$78.26	5.564%	$82.6144	$82.61	$82.60	$82.60
7 purchase totals =	$592.86		$643.2572	$643.26	$643.15	$643.30