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	$18.99	11.670%	$21.2061	$21.21	$21.20	$21.20
Purchase #2	$142.51	6.175%	$151.31	$151.31	$151.30	$151.30
Purchase #3	$25.62	8.735%	$27.8579	$27.86	$27.85	$27.90
Purchase #4	$190.69	11.448%	$212.5202	$212.52	$212.50	$212.50
Purchase #5	$45.33	11.534%	$50.5584	$50.56	$50.55	$50.60
Purchase #6	$174.25	3.134%	$179.711	$179.71	$179.70	$179.70
Purchase #7	$232.81	7.534%	$250.3499	$250.35	$250.35	$250.30
7 purchase totals =	$830.20		$893.5135	$893.52	$893.45	$893.50