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	$25.42	8.175%	$27.4981	$27.50	$27.50	$27.50
Purchase #2	$99.69	3.594%	$103.2729	$103.27	$103.25	$103.30
Purchase #3	$127.90	7.771%	$137.8391	$137.84	$137.85	$137.80
Purchase #4	$158.36	3.744%	$164.289	$164.29	$164.30	$164.30
Purchase #5	$210.61	13.671%	$239.4025	$239.40	$239.40	$239.40
Purchase #6	$11.27	4.872%	$11.8191	$11.82	$11.80	$11.80
Purchase #7	$139.63	11.795%	$156.0994	$156.10	$156.10	$156.10
7 purchase totals =	$772.88		$840.2201	$840.22	$840.20	$840.20