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	$18.53	13.244%	$20.9841	$20.98	$21.00	$21.00
Purchase #2	$42.32	3.370%	$43.7462	$43.75	$43.75	$43.70
Purchase #3	$216.72	9.424%	$237.1437	$237.14	$237.15	$237.10
Purchase #4	$72.77	10.783%	$80.6168	$80.62	$80.60	$80.60
Purchase #5	$85.60	5.566%	$90.3645	$90.36	$90.35	$90.40
Purchase #6	$144.96	10.388%	$160.0184	$160.02	$160.00	$160.00
Purchase #7	$105.29	12.421%	$118.3681	$118.37	$118.35	$118.40
7 purchase totals =	$686.19		$751.2418	$751.24	$751.20	$751.20