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	$74.39	7.565%	$80.0176	$80.02	$80.00	$80.00
Purchase #2	$139.66	4.269%	$145.6221	$145.62	$145.60	$145.60
Purchase #3	$82.95	9.436%	$90.7772	$90.78	$90.80	$90.80
Purchase #4	$156.34	9.272%	$170.8358	$170.84	$170.85	$170.80
Purchase #5	$39.68	13.741%	$45.1324	$45.13	$45.15	$45.10
Purchase #6	$210.39	4.248%	$219.3274	$219.33	$219.35	$219.30
Purchase #7	$66.42	12.180%	$74.51	$74.51	$74.50	$74.50
7 purchase totals =	$769.83		$826.2225	$826.23	$826.25	$826.10