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	$16.93	10.794%	$18.7574	$18.76	$18.75	$18.80
Purchase #2	$243.85	9.854%	$267.879	$267.88	$267.90	$267.90
Purchase #3	$162.89	9.799%	$178.8516	$178.85	$178.85	$178.90
Purchase #4	$169.60	9.401%	$185.5441	$185.54	$185.55	$185.50
Purchase #5	$32.42	6.541%	$34.5406	$34.54	$34.55	$34.50
Purchase #6	$75.73	13.437%	$85.9058	$85.91	$85.90	$85.90
Purchase #7	$92.31	7.532%	$99.2628	$99.26	$99.25	$99.30
7 purchase totals =	$793.73		$870.7413	$870.74	$870.75	$870.80