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	$17.76	12.473%	$19.9752	$19.98	$20.00	$20.00
Purchase #2	$225.95	11.325%	$251.5388	$251.54	$251.55	$251.50
Purchase #3	$158.34	6.446%	$168.5466	$168.55	$168.55	$168.50
Purchase #4	$237.28	11.406%	$264.3442	$264.34	$264.35	$264.30
Purchase #5	$53.55	8.151%	$57.9149	$57.91	$57.90	$57.90
Purchase #6	$70.28	5.205%	$73.9381	$73.94	$73.95	$73.90
Purchase #7	$157.86	4.139%	$164.3938	$164.39	$164.40	$164.40
7 purchase totals =	$921.02		$1,000.6516	$1,000.65	$1,000.70	$1,000.50