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	$18.87	13.506%	$21.4186	$21.42	$21.40	$21.40
Purchase #2	$57.96	9.677%	$63.5688	$63.57	$63.55	$63.60
Purchase #3	$226.36	10.780%	$250.7616	$250.76	$250.75	$250.80
Purchase #4	$150.94	6.257%	$160.3843	$160.38	$160.40	$160.40
Purchase #5	$236.91	8.459%	$256.9502	$256.95	$256.95	$257.00
Purchase #6	$101.30	7.872%	$109.2743	$109.27	$109.25	$109.30
Purchase #7	$54.72	8.690%	$59.4752	$59.48	$59.50	$59.50
7 purchase totals =	$847.06		$921.8330	$921.83	$921.80	$922.00