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 $6.80   9.108%   $7.4193   $7.42   $7.40   $7.40  
  Purchase #2 $138.36   12.429%   $155.5568   $155.56   $155.55   $155.60  
  Purchase #3 $190.76   4.411%   $199.1744   $199.17   $199.15   $199.20  
  Purchase #4 $159.49   7.669%   $171.7213   $171.72   $171.70   $171.70  
  Purchase #5 $93.71   7.749%   $100.9716   $100.97   $100.95   $101.00  
  Purchase #6 $72.57   9.687%   $79.5999   $79.60   $79.60   $79.60  
  Purchase #7 $133.66   9.143%   $145.8805   $145.88   $145.90   $145.90  
  7 purchase totals = $795.35       $860.3238   $860.32   $860.25   $860.40