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	$19.30	6.266%	$20.5093	$20.51	$20.50	$20.50
Purchase #2	$155.84	8.294%	$168.7654	$168.77	$168.75	$168.80
Purchase #3	$131.62	8.600%	$142.9393	$142.94	$142.95	$142.90
Purchase #4	$239.85	13.272%	$271.6829	$271.68	$271.70	$271.70
Purchase #5	$109.59	13.798%	$124.7112	$124.71	$124.70	$124.70
Purchase #6	$41.52	5.534%	$43.8177	$43.82	$43.80	$43.80
Purchase #7	$27.54	3.231%	$28.4298	$28.43	$28.45	$28.40
7 purchase totals =	$725.26		$800.8556	$800.86	$800.85	$800.80