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	$25.77	8.582%	$27.9816	$27.98	$28.00	$28.00
Purchase #2	$122.39	13.507%	$138.9212	$138.92	$138.90	$138.90
Purchase #3	$104.36	3.415%	$107.9239	$107.92	$107.90	$107.90
Purchase #4	$78.39	13.349%	$88.8543	$88.85	$88.85	$88.90
Purchase #5	$123.56	5.681%	$130.5794	$130.58	$130.60	$130.60
Purchase #6	$75.52	3.702%	$78.3158	$78.32	$78.30	$78.30
Purchase #7	$155.61	10.104%	$171.3328	$171.33	$171.35	$171.30
7 purchase totals =	$685.60		$743.9090	$743.90	$743.90	$743.90