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	$242.25	12.813%	$273.2895	$273.29	$273.30	$273.30
Purchase #2	$94.41	3.370%	$97.5916	$97.59	$97.60	$97.60
Purchase #3	$105.94	7.495%	$113.8802	$113.88	$113.90	$113.90
Purchase #4	$12.57	6.489%	$13.3857	$13.39	$13.40	$13.40
Purchase #5	$115.79	10.504%	$127.9526	$127.95	$127.95	$128.00
Purchase #6	$78.58	9.254%	$85.8518	$85.85	$85.85	$85.90
Purchase #7	$95.28	13.269%	$107.9227	$107.92	$107.90	$107.90
7 purchase totals =	$744.82		$819.8741	$819.87	$819.90	$820.00