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	$19.44	10.185%	$21.42	$21.42	$21.40	$21.40
Purchase #2	$123.96	9.649%	$135.9209	$135.92	$135.90	$135.90
Purchase #3	$249.72	4.299%	$260.4555	$260.46	$260.45	$260.50
Purchase #4	$109.61	7.500%	$117.8308	$117.83	$117.85	$117.80
Purchase #5	$61.59	9.764%	$67.6036	$67.60	$67.60	$67.60
Purchase #6	$95.50	8.539%	$103.6547	$103.65	$103.65	$103.70
Purchase #7	$131.51	9.762%	$144.348	$144.35	$144.35	$144.30
7 purchase totals =	$791.33		$851.2335	$851.23	$851.20	$851.20