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	$120.96	12.706%	$136.3292	$136.33	$136.35	$136.30
Purchase #2	$107.65	6.543%	$114.6935	$114.69	$114.70	$114.70
Purchase #3	$88.87	3.374%	$91.8685	$91.87	$91.85	$91.90
Purchase #4	$25.66	12.810%	$28.947	$28.95	$28.95	$28.90
Purchase #5	$19.33	7.211%	$20.7239	$20.72	$20.70	$20.70
Purchase #6	$89.98	9.573%	$98.5938	$98.59	$98.60	$98.60
Purchase #7	$168.31	4.396%	$175.7089	$175.71	$175.70	$175.70
7 purchase totals =	$620.76		$666.8648	$666.86	$666.85	$666.80