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	$18.80	10.771%	$20.8249	$20.82	$20.80	$20.80
Purchase #2	$171.70	9.466%	$187.9531	$187.95	$187.95	$188.00
Purchase #3	$72.72	12.344%	$81.6966	$81.70	$81.70	$81.70
Purchase #4	$38.31	9.425%	$41.9207	$41.92	$41.90	$41.90
Purchase #5	$91.88	7.784%	$99.0319	$99.03	$99.05	$99.00
Purchase #6	$161.25	3.390%	$166.7164	$166.72	$166.70	$166.70
Purchase #7	$38.52	7.527%	$41.4194	$41.42	$41.40	$41.40
7 purchase totals =	$593.18		$639.5630	$639.56	$639.50	$639.50