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	$36.58	9.392%	$40.0156	$40.02	$40.00	$40.00
Purchase #2	$180.81	8.557%	$196.2819	$196.28	$196.30	$196.30
Purchase #3	$59.27	7.608%	$63.7793	$63.78	$63.80	$63.80
Purchase #4	$114.39	13.130%	$129.4094	$129.41	$129.40	$129.40
Purchase #5	$72.45	8.577%	$78.664	$78.66	$78.65	$78.70
Purchase #6	$60.45	11.613%	$67.4701	$67.47	$67.45	$67.50
Purchase #7	$13.79	7.729%	$14.8558	$14.86	$14.85	$14.90
7 purchase totals =	$537.74		$590.4761	$590.48	$590.45	$590.60