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 $38.54   4.335%   $40.2107   $40.21   $40.20   $40.20  
  Purchase #2 $29.59   6.645%   $31.5563   $31.56   $31.55   $31.60  
  Purchase #3 $142.89   4.346%   $149.1   $149.10   $149.10   $149.10  
  Purchase #4 $137.48   7.808%   $148.2144   $148.21   $148.20   $148.20  
  Purchase #5 $143.63   3.843%   $149.1497   $149.15   $149.15   $149.10  
  Purchase #6 $233.52   12.477%   $262.6563   $262.66   $262.65   $262.70  
  Purchase #7 $244.42   5.354%   $257.5062   $257.51   $257.50   $257.50  
  7 purchase totals = $970.07       $1,038.3936   $1,038.40   $1,038.35   $1,038.40