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	$18.40	10.264%	$20.2886	$20.29	$20.30	$20.30
Purchase #2	$98.77	10.272%	$108.9157	$108.92	$108.90	$108.90
Purchase #3	$38.35	4.128%	$39.9331	$39.93	$39.95	$39.90
Purchase #4	$81.25	10.223%	$89.5562	$89.56	$89.55	$89.60
Purchase #5	$63.97	6.105%	$67.8754	$67.88	$67.90	$67.90
Purchase #6	$197.85	10.737%	$219.0932	$219.09	$219.10	$219.10
Purchase #7	$112.61	11.829%	$125.9306	$125.93	$125.95	$125.90
7 purchase totals =	$611.20		$671.5928	$671.60	$671.65	$671.60