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	$18.34	13.837%	$20.8777	$20.88	$20.90	$20.90
Purchase #2	$214.41	11.406%	$238.8656	$238.87	$238.85	$238.90
Purchase #3	$238.92	11.301%	$265.9203	$265.92	$265.90	$265.90
Purchase #4	$189.37	11.212%	$210.6022	$210.60	$210.60	$210.60
Purchase #5	$19.78	11.251%	$22.0054	$22.01	$22.00	$22.00
Purchase #6	$84.73	5.459%	$89.3554	$89.36	$89.35	$89.40
Purchase #7	$108.92	13.474%	$123.5959	$123.60	$123.60	$123.60
7 purchase totals =	$874.47		$971.2225	$971.24	$971.20	$971.30