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	$18.69	12.815%	$21.0851	$21.09	$21.10	$21.10
Purchase #2	$164.51	4.255%	$171.5099	$171.51	$171.50	$171.50
Purchase #3	$206.54	7.394%	$221.8116	$221.81	$221.80	$221.80
Purchase #4	$109.58	9.583%	$120.0811	$120.08	$120.10	$120.10
Purchase #5	$81.71	10.274%	$90.1049	$90.10	$90.10	$90.10
Purchase #6	$54.34	9.170%	$59.323	$59.32	$59.30	$59.30
Purchase #7	$86.94	5.133%	$91.4026	$91.40	$91.40	$91.40
7 purchase totals =	$722.31		$775.3182	$775.31	$775.30	$775.30