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	$36.40	9.843%	$39.9829	$39.98	$40.00	$40.00
Purchase #2	$142.56	12.659%	$160.6067	$160.61	$160.60	$160.60
Purchase #3	$127.79	10.852%	$141.6578	$141.66	$141.65	$141.70
Purchase #4	$193.94	3.694%	$201.1041	$201.10	$201.10	$201.10
Purchase #5	$52.96	12.710%	$59.6912	$59.69	$59.70	$59.70
Purchase #6	$59.49	13.272%	$67.3855	$67.39	$67.40	$67.40
Purchase #7	$168.77	9.178%	$184.2597	$184.26	$184.25	$184.30
7 purchase totals =	$781.91		$854.6879	$854.69	$854.70	$854.80