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	$35.32	13.223%	$39.9904	$39.99	$40.00	$40.00
Purchase #2	$27.67	13.471%	$31.3974	$31.40	$31.40	$31.40
Purchase #3	$237.33	5.595%	$250.6086	$250.61	$250.60	$250.60
Purchase #4	$167.96	6.511%	$178.8959	$178.90	$178.90	$178.90
Purchase #5	$168.56	3.815%	$174.9906	$174.99	$175.00	$175.00
Purchase #6	$84.56	6.538%	$90.0885	$90.09	$90.10	$90.10
Purchase #7	$50.32	4.712%	$52.6911	$52.69	$52.70	$52.70
7 purchase totals =	$771.72		$818.6625	$818.67	$818.70	$818.70