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	$18.91	12.773%	$21.3254	$21.33	$21.35	$21.30
Purchase #2	$81.99	11.499%	$91.418	$91.42	$91.40	$91.40
Purchase #3	$64.93	6.207%	$68.9602	$68.96	$68.95	$69.00
Purchase #4	$184.81	3.695%	$191.6387	$191.64	$191.65	$191.60
Purchase #5	$26.84	10.241%	$29.5887	$29.59	$29.60	$29.60
Purchase #6	$70.50	11.395%	$78.5335	$78.53	$78.55	$78.50
Purchase #7	$37.89	12.461%	$42.6115	$42.61	$42.60	$42.60
7 purchase totals =	$485.87		$524.0760	$524.08	$524.10	$524.00