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	$18.76	6.577%	$19.9938	$19.99	$20.00	$20.00
Purchase #2	$90.86	5.367%	$95.7365	$95.74	$95.75	$95.70
Purchase #3	$164.34	10.346%	$181.3426	$181.34	$181.35	$181.30
Purchase #4	$243.51	8.645%	$264.5614	$264.56	$264.55	$264.60
Purchase #5	$18.39	10.584%	$20.3364	$20.34	$20.35	$20.30
Purchase #6	$72.37	13.585%	$82.2015	$82.20	$82.20	$82.20
Purchase #7	$172.48	6.755%	$184.131	$184.13	$184.15	$184.10
7 purchase totals =	$780.71		$848.3032	$848.30	$848.35	$848.20