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	$5.85	12.809%	$6.5993	$6.60	$6.60	$6.60
Purchase #2	$112.61	9.724%	$123.5602	$123.56	$123.55	$123.60
Purchase #3	$34.34	8.531%	$37.2695	$37.27	$37.25	$37.30
Purchase #4	$97.85	8.662%	$106.3258	$106.33	$106.35	$106.30
Purchase #5	$17.80	9.765%	$19.5382	$19.54	$19.55	$19.50
Purchase #6	$187.88	11.656%	$209.7793	$209.78	$209.80	$209.80
Purchase #7	$68.72	5.499%	$72.4989	$72.50	$72.50	$72.50
7 purchase totals =	$525.05		$575.5712	$575.58	$575.60	$575.60