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	$7.54	11.393%	$8.399	$8.40	$8.40	$8.40
Purchase #2	$98.67	11.345%	$109.8641	$109.86	$109.85	$109.90
Purchase #3	$136.60	5.355%	$143.9149	$143.91	$143.90	$143.90
Purchase #4	$156.37	11.523%	$174.3885	$174.39	$174.40	$174.40
Purchase #5	$58.96	9.280%	$64.4315	$64.43	$64.45	$64.40
Purchase #6	$157.83	8.708%	$171.5738	$171.57	$171.55	$171.60
Purchase #7	$142.67	10.639%	$157.8487	$157.85	$157.85	$157.80
7 purchase totals =	$758.64		$830.4205	$830.41	$830.40	$830.40