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	$34.34	13.735%	$39.0566	$39.06	$39.05	$39.10
Purchase #2	$28.36	9.669%	$31.1021	$31.10	$31.10	$31.10
Purchase #3	$173.51	10.488%	$191.7077	$191.71	$191.70	$191.70
Purchase #4	$86.53	4.745%	$90.6358	$90.64	$90.65	$90.60
Purchase #5	$226.63	12.126%	$254.1112	$254.11	$254.10	$254.10
Purchase #6	$133.80	9.280%	$146.2166	$146.22	$146.20	$146.20
Purchase #7	$179.48	9.206%	$196.0029	$196.00	$196.00	$196.00
7 purchase totals =	$862.65		$948.8329	$948.84	$948.80	$948.80