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	$18.80	13.591%	$21.3551	$21.36	$21.35	$21.40
Purchase #2	$71.92	11.827%	$80.426	$80.43	$80.45	$80.40
Purchase #3	$98.80	13.458%	$112.0965	$112.10	$112.10	$112.10
Purchase #4	$142.35	3.500%	$147.3323	$147.33	$147.35	$147.30
Purchase #5	$85.52	6.235%	$90.8522	$90.85	$90.85	$90.90
Purchase #6	$92.38	10.818%	$102.3737	$102.37	$102.35	$102.40
Purchase #7	$224.88	9.153%	$245.4633	$245.46	$245.45	$245.50
7 purchase totals =	$734.65		$799.8991	$799.90	$799.90	$800.00