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	$249.29	12.119%	$279.5015	$279.50	$279.50	$279.50
Purchase #2	$6.29	13.630%	$7.1473	$7.15	$7.15	$7.10
Purchase #3	$167.59	10.137%	$184.5786	$184.58	$184.60	$184.60
Purchase #4	$31.69	4.146%	$33.0039	$33.00	$33.00	$33.00
Purchase #5	$48.84	10.757%	$54.0937	$54.09	$54.10	$54.10
Purchase #6	$25.95	7.550%	$27.9092	$27.91	$27.90	$27.90
Purchase #7	$226.86	3.213%	$234.149	$234.15	$234.15	$234.10
7 purchase totals =	$756.51		$820.3832	$820.38	$820.40	$820.30