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	$34.89	12.516%	$39.2568	$39.26	$39.25	$39.30
Purchase #2	$164.61	4.405%	$171.8611	$171.86	$171.85	$171.90
Purchase #3	$59.71	9.564%	$65.4207	$65.42	$65.40	$65.40
Purchase #4	$125.28	13.399%	$142.0663	$142.07	$142.05	$142.10
Purchase #5	$36.29	12.718%	$40.9054	$40.91	$40.90	$40.90
Purchase #6	$145.58	13.449%	$165.1591	$165.16	$165.15	$165.20
Purchase #7	$234.38	8.478%	$254.2507	$254.25	$254.25	$254.30
7 purchase totals =	$800.74		$878.9201	$878.93	$878.85	$879.10