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 $135.69   3.455%   $140.3781   $140.38   $140.40   $140.40  
  Purchase #2 $15.71   13.376%   $17.8114   $17.81   $17.80   $17.80  
  Purchase #3 $198.44   13.183%   $224.6003   $224.60   $224.60   $224.60  
  Purchase #4 $126.81   7.816%   $136.7215   $136.72   $136.70   $136.70  
  Purchase #5 $63.27   9.133%   $69.0484   $69.05   $69.05   $69.00  
  Purchase #6 $134.48   6.750%   $143.5574   $143.56   $143.55   $143.60  
  Purchase #7 $193.78   3.224%   $200.0275   $200.03   $200.05   $200.00  
  7 purchase totals = $868.18       $932.1446   $932.15   $932.15   $932.10