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	$5.92	11.263%	$6.5868	$6.59	$6.60	$6.60
Purchase #2	$90.72	9.172%	$99.0408	$99.04	$99.05	$99.00
Purchase #3	$174.88	10.648%	$193.5012	$193.50	$193.50	$193.50
Purchase #4	$236.69	7.801%	$255.1542	$255.15	$255.15	$255.20
Purchase #5	$100.55	9.232%	$109.8328	$109.83	$109.85	$109.80
Purchase #6	$216.48	13.348%	$245.3758	$245.38	$245.40	$245.40
Purchase #7	$198.29	7.689%	$213.5365	$213.54	$213.55	$213.50
7 purchase totals =	$1,023.53		$1,123.0281	$1,123.03	$1,123.10	$1,123.00