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	$37.70	3.579%	$39.0493	$39.05	$39.05	$39.00
Purchase #2	$184.55	4.603%	$193.0448	$193.04	$193.05	$193.00
Purchase #3	$176.83	3.738%	$183.4399	$183.44	$183.45	$183.40
Purchase #4	$46.86	7.464%	$50.3576	$50.36	$50.35	$50.40
Purchase #5	$147.72	11.646%	$164.9235	$164.92	$164.90	$164.90
Purchase #6	$89.40	12.200%	$100.3068	$100.31	$100.30	$100.30
Purchase #7	$40.33	12.397%	$45.3297	$45.33	$45.35	$45.30
7 purchase totals =	$723.39		$776.4516	$776.45	$776.45	$776.30