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	$5.91	7.732%	$6.367	$6.37	$6.35	$6.40
Purchase #2	$91.33	13.393%	$103.5618	$103.56	$103.55	$103.60
Purchase #3	$127.28	9.751%	$139.6911	$139.69	$139.70	$139.70
Purchase #4	$123.52	4.167%	$128.6671	$128.67	$128.65	$128.70
Purchase #5	$201.61	8.264%	$218.2711	$218.27	$218.25	$218.30
Purchase #6	$54.68	12.813%	$61.6861	$61.69	$61.70	$61.70
Purchase #7	$12.41	9.476%	$13.586	$13.59	$13.60	$13.60
7 purchase totals =	$616.74		$671.8302	$671.84	$671.80	$672.00