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	$20.40	3.168%	$21.0463	$21.05	$21.05	$21.00
Purchase #2	$36.76	11.360%	$40.9359	$40.94	$40.95	$40.90
Purchase #3	$10.41	8.401%	$11.2845	$11.28	$11.30	$11.30
Purchase #4	$90.33	7.856%	$97.4263	$97.43	$97.45	$97.40
Purchase #5	$84.73	9.291%	$92.6023	$92.60	$92.60	$92.60
Purchase #6	$82.97	9.853%	$91.145	$91.15	$91.15	$91.10
Purchase #7	$186.71	10.817%	$206.9064	$206.91	$206.90	$206.90
7 purchase totals =	$512.31		$561.3467	$561.36	$561.40	$561.20