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	$19.76	3.787%	$20.5083	$20.51	$20.50	$20.50
Purchase #2	$90.86	13.812%	$103.4096	$103.41	$103.40	$103.40
Purchase #3	$150.53	13.717%	$171.1782	$171.18	$171.20	$171.20
Purchase #4	$61.29	11.787%	$68.5143	$68.51	$68.50	$68.50
Purchase #5	$144.42	11.242%	$160.6557	$160.66	$160.65	$160.70
Purchase #6	$218.46	8.871%	$237.8396	$237.84	$237.85	$237.80
Purchase #7	$74.39	8.588%	$80.7786	$80.78	$80.80	$80.80
7 purchase totals =	$759.71		$842.8843	$842.89	$842.90	$842.90