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 $55.70   8.788%   $60.5949   $60.59   $60.60   $60.60  
  Purchase #2 $232.36   9.295%   $253.9579   $253.96   $253.95   $254.00  
  Purchase #3 $196.47   13.418%   $222.8323   $222.83   $222.85   $222.80  
  Purchase #4 $232.77   6.580%   $248.0863   $248.09   $248.10   $248.10  
  Purchase #5 $93.63   10.623%   $103.5763   $103.58   $103.60   $103.60  
  Purchase #6 $14.79   5.669%   $15.6284   $15.63   $15.65   $15.60  
  Purchase #7 $86.43   10.184%   $95.232   $95.23   $95.25   $95.20  
  7 purchase totals = $912.15       $999.9081   $999.91   $1,000.00   $999.90