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	$39.57	6.747%	$42.2398	$42.24	$42.25	$42.20
Purchase #2	$37.32	4.160%	$38.8725	$38.87	$38.85	$38.90
Purchase #3	$113.34	12.560%	$127.5755	$127.58	$127.60	$127.60
Purchase #4	$58.27	12.533%	$65.573	$65.57	$65.55	$65.60
Purchase #5	$179.76	5.238%	$189.1758	$189.18	$189.20	$189.20
Purchase #6	$96.97	9.655%	$106.3325	$106.33	$106.35	$106.30
Purchase #7	$61.70	11.628%	$68.8745	$68.87	$68.85	$68.90
7 purchase totals =	$586.93		$638.6436	$638.64	$638.65	$638.70