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	$79.91	12.581%	$89.9635	$89.96	$89.95	$90.00
Purchase #2	$242.61	12.852%	$273.7902	$273.79	$273.80	$273.80
Purchase #3	$65.91	10.103%	$72.5689	$72.57	$72.55	$72.60
Purchase #4	$102.71	3.673%	$106.4825	$106.48	$106.50	$106.50
Purchase #5	$77.49	13.250%	$87.7574	$87.76	$87.75	$87.80
Purchase #6	$49.73	6.190%	$52.8083	$52.81	$52.80	$52.80
Purchase #7	$152.41	9.685%	$167.1709	$167.17	$167.15	$167.20
7 purchase totals =	$770.77		$850.5417	$850.54	$850.50	$850.70