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	$5.97	11.824%	$6.6759	$6.68	$6.70	$6.70
Purchase #2	$213.92	10.716%	$236.8437	$236.84	$236.85	$236.80
Purchase #3	$85.40	4.717%	$89.4283	$89.43	$89.45	$89.40
Purchase #4	$55.37	8.175%	$59.8965	$59.90	$59.90	$59.90
Purchase #5	$52.40	9.118%	$57.1778	$57.18	$57.20	$57.20
Purchase #6	$78.52	4.794%	$82.2842	$82.28	$82.30	$82.30
Purchase #7	$208.97	4.333%	$218.0247	$218.02	$218.00	$218.00
7 purchase totals =	$700.55		$750.3311	$750.33	$750.40	$750.30