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	$246.46	12.454%	$277.1541	$277.15	$277.15	$277.20
Purchase #2	$127.45	7.250%	$136.6901	$136.69	$136.70	$136.70
Purchase #3	$101.89	10.428%	$112.5151	$112.52	$112.50	$112.50
Purchase #4	$31.31	5.798%	$33.1254	$33.13	$33.15	$33.10
Purchase #5	$83.30	6.305%	$88.5521	$88.55	$88.55	$88.60
Purchase #6	$128.42	8.651%	$139.5296	$139.53	$139.55	$139.50
Purchase #7	$48.73	8.725%	$52.9817	$52.98	$53.00	$53.00
7 purchase totals =	$767.56		$840.5481	$840.55	$840.60	$840.60