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	$84.34	6.590%	$89.898	$89.90	$89.90	$89.90
Purchase #2	$121.74	7.715%	$131.1322	$131.13	$131.15	$131.10
Purchase #3	$37.59	12.698%	$42.3632	$42.36	$42.35	$42.40
Purchase #4	$118.25	8.176%	$127.9181	$127.92	$127.90	$127.90
Purchase #5	$58.37	9.727%	$64.0476	$64.05	$64.05	$64.00
Purchase #6	$202.50	6.135%	$214.9234	$214.92	$214.90	$214.90
Purchase #7	$178.51	4.797%	$187.0731	$187.07	$187.05	$187.10
7 purchase totals =	$801.30		$857.3556	$857.35	$857.30	$857.30