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	$249.63	12.109%	$279.8577	$279.86	$279.85	$279.90
Purchase #2	$70.41	11.442%	$78.4663	$78.47	$78.45	$78.50
Purchase #3	$109.85	5.207%	$115.5699	$115.57	$115.55	$115.60
Purchase #4	$113.34	9.545%	$124.1583	$124.16	$124.15	$124.20
Purchase #5	$165.81	3.290%	$171.2651	$171.27	$171.25	$171.30
Purchase #6	$108.47	12.853%	$122.4116	$122.41	$122.40	$122.40
Purchase #7	$94.40	11.431%	$105.1909	$105.19	$105.20	$105.20
7 purchase totals =	$911.91		$996.9198	$996.93	$996.85	$997.10