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	$79.74	12.234%	$89.4954	$89.50	$89.50	$89.50
Purchase #2	$160.71	13.303%	$182.0893	$182.09	$182.10	$182.10
Purchase #3	$132.98	9.284%	$145.3259	$145.33	$145.35	$145.30
Purchase #4	$187.53	3.766%	$194.5924	$194.59	$194.60	$194.60
Purchase #5	$82.92	8.682%	$90.1191	$90.12	$90.10	$90.10
Purchase #6	$72.93	12.141%	$81.7844	$81.78	$81.80	$81.80
Purchase #7	$41.27	13.458%	$46.8241	$46.82	$46.80	$46.80
7 purchase totals =	$758.08		$830.2306	$830.23	$830.25	$830.20