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	$249.36	12.191%	$279.7595	$279.76	$279.75	$279.80
Purchase #2	$144.82	13.786%	$164.7849	$164.78	$164.80	$164.80
Purchase #3	$113.51	12.734%	$127.9644	$127.96	$127.95	$128.00
Purchase #4	$112.65	5.499%	$118.8446	$118.84	$118.85	$118.80
Purchase #5	$197.32	3.833%	$204.8833	$204.88	$204.90	$204.90
Purchase #6	$177.33	4.591%	$185.4712	$185.47	$185.45	$185.50
Purchase #7	$120.86	13.202%	$136.8159	$136.82	$136.80	$136.80
7 purchase totals =	$1,115.85		$1,218.5238	$1,218.51	$1,218.50	$1,218.60