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 $125.94   10.377%   $139.0088   $139.01   $139.00   $139.00  
  Purchase #2 $227.59   11.462%   $253.6764   $253.68   $253.70   $253.70  
  Purchase #3 $55.53   13.766%   $63.1743   $63.17   $63.15   $63.20  
  Purchase #4 $79.76   8.381%   $86.4447   $86.44   $86.45   $86.40  
  Purchase #5 $101.61   8.842%   $110.5944   $110.59   $110.60   $110.60  
  Purchase #6 $191.83   13.591%   $217.9016   $217.90   $217.90   $217.90  
  Purchase #7 $142.31   7.217%   $152.5805   $152.58   $152.60   $152.60  
  7 purchase totals = $924.57       $1,023.3807   $1,023.37   $1,023.40   $1,023.40