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	$8.69	12.666%	$9.7907	$9.79	$9.80	$9.80
Purchase #2	$50.85	7.586%	$54.7075	$54.71	$54.70	$54.70
Purchase #3	$34.65	12.276%	$38.9036	$38.90	$38.90	$38.90
Purchase #4	$167.54	11.292%	$186.4586	$186.46	$186.45	$186.50
Purchase #5	$12.89	13.583%	$14.6408	$14.64	$14.65	$14.60
Purchase #6	$229.85	8.109%	$248.4885	$248.49	$248.50	$248.50
Purchase #7	$79.25	9.126%	$86.4824	$86.48	$86.50	$86.50
7 purchase totals =	$583.72		$639.4721	$639.47	$639.50	$639.50