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	$24.62	13.233%	$27.878	$27.88	$27.90	$27.90
Purchase #2	$235.75	11.718%	$263.3752	$263.38	$263.40	$263.40
Purchase #3	$105.61	7.554%	$113.5878	$113.59	$113.60	$113.60
Purchase #4	$31.38	3.807%	$32.5746	$32.57	$32.55	$32.60
Purchase #5	$67.48	8.148%	$72.9783	$72.98	$73.00	$73.00
Purchase #6	$37.79	13.609%	$42.9328	$42.93	$42.95	$42.90
Purchase #7	$95.44	6.145%	$101.3048	$101.30	$101.30	$101.30
7 purchase totals =	$598.07		$654.6315	$654.63	$654.70	$654.70