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	$26.91	11.116%	$29.9013	$29.90	$29.90	$29.90
Purchase #2	$177.47	7.249%	$190.3348	$190.33	$190.35	$190.30
Purchase #3	$172.54	3.450%	$178.4926	$178.49	$178.50	$178.50
Purchase #4	$94.68	9.650%	$103.8166	$103.82	$103.80	$103.80
Purchase #5	$81.93	10.781%	$90.7629	$90.76	$90.75	$90.80
Purchase #6	$165.45	3.801%	$171.7388	$171.74	$171.75	$171.70
Purchase #7	$26.49	8.106%	$28.6373	$28.64	$28.65	$28.60
7 purchase totals =	$745.47		$793.6843	$793.68	$793.70	$793.60