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	$87.44	3.386%	$90.4007	$90.40	$90.40	$90.40
Purchase #2	$55.76	5.645%	$58.9077	$58.91	$58.90	$58.90
Purchase #3	$181.81	8.317%	$196.9311	$196.93	$196.95	$196.90
Purchase #4	$24.89	9.653%	$27.2926	$27.29	$27.30	$27.30
Purchase #5	$81.43	12.480%	$91.5925	$91.59	$91.60	$91.60
Purchase #6	$18.77	4.165%	$19.5518	$19.55	$19.55	$19.60
Purchase #7	$98.88	13.111%	$111.8442	$111.84	$111.85	$111.80
7 purchase totals =	$548.98		$596.5206	$596.51	$596.55	$596.50