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	$77.49	3.869%	$80.4881	$80.49	$80.50	$80.50
Purchase #2	$137.31	6.789%	$146.632	$146.63	$146.65	$146.60
Purchase #3	$6.52	6.159%	$6.9216	$6.92	$6.90	$6.90
Purchase #4	$92.30	13.661%	$104.9091	$104.91	$104.90	$104.90
Purchase #5	$201.62	10.761%	$223.3163	$223.32	$223.30	$223.30
Purchase #6	$199.59	11.693%	$222.9281	$222.93	$222.95	$222.90
Purchase #7	$50.65	12.285%	$56.8724	$56.87	$56.85	$56.90
7 purchase totals =	$765.48		$842.0676	$842.07	$842.05	$842.00