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	$90.91	10.181%	$100.1655	$100.17	$100.15	$100.20
Purchase #2	$141.94	12.736%	$160.0175	$160.02	$160.00	$160.00
Purchase #3	$64.56	12.261%	$72.4757	$72.48	$72.50	$72.50
Purchase #4	$95.97	3.431%	$99.2627	$99.26	$99.25	$99.30
Purchase #5	$179.52	9.136%	$195.9209	$195.92	$195.90	$195.90
Purchase #6	$102.53	6.720%	$109.42	$109.42	$109.40	$109.40
Purchase #7	$91.96	12.219%	$103.1966	$103.20	$103.20	$103.20
7 purchase totals =	$767.39		$840.4589	$840.47	$840.40	$840.50