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	$39.46	8.239%	$42.7111	$42.71	$42.70	$42.70
Purchase #2	$80.74	4.156%	$84.0956	$84.10	$84.10	$84.10
Purchase #3	$190.50	4.207%	$198.5143	$198.51	$198.50	$198.50
Purchase #4	$239.67	13.106%	$271.0812	$271.08	$271.10	$271.10
Purchase #5	$35.80	4.565%	$37.4343	$37.43	$37.45	$37.40
Purchase #6	$37.68	12.316%	$42.3207	$42.32	$42.30	$42.30
Purchase #7	$131.48	9.709%	$144.2454	$144.25	$144.25	$144.20
7 purchase totals =	$755.33		$820.4026	$820.40	$820.40	$820.30