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	$24.32	13.255%	$27.5436	$27.54	$27.55	$27.50
Purchase #2	$101.62	6.596%	$108.3229	$108.32	$108.30	$108.30
Purchase #3	$40.28	6.427%	$42.8688	$42.87	$42.85	$42.90
Purchase #4	$33.69	10.604%	$37.2625	$37.26	$37.25	$37.30
Purchase #5	$137.50	3.123%	$141.7941	$141.79	$141.80	$141.80
Purchase #6	$230.62	7.623%	$248.2002	$248.20	$248.20	$248.20
Purchase #7	$37.76	8.474%	$40.9598	$40.96	$40.95	$41.00
7 purchase totals =	$605.79		$646.9519	$646.94	$646.90	$647.00