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	$20.37	4.199%	$21.2253	$21.23	$21.25	$21.20
Purchase #2	$168.57	12.495%	$189.6328	$189.63	$189.65	$189.60
Purchase #3	$113.90	12.340%	$127.9553	$127.96	$127.95	$128.00
Purchase #4	$67.58	5.841%	$71.5273	$71.53	$71.55	$71.50
Purchase #5	$15.50	12.832%	$17.489	$17.49	$17.50	$17.50
Purchase #6	$133.90	6.235%	$142.2487	$142.25	$142.25	$142.20
Purchase #7	$246.82	9.612%	$270.5443	$270.54	$270.55	$270.50
7 purchase totals =	$766.64		$840.6227	$840.63	$840.70	$840.50