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 $8.39   9.320%   $9.1719   $9.17   $9.15   $9.20  
  Purchase #2 $45.75   9.755%   $50.2129   $50.21   $50.20   $50.20  
  Purchase #3 $27.67   5.700%   $29.2472   $29.25   $29.25   $29.20  
  Purchase #4 $105.78   9.362%   $115.6831   $115.68   $115.70   $115.70  
  Purchase #5 $242.53   6.870%   $259.1918   $259.19   $259.20   $259.20  
  Purchase #6 $45.70   8.389%   $49.5338   $49.53   $49.55   $49.50  
  Purchase #7 $78.26   3.240%   $80.7956   $80.80   $80.80   $80.80  
  7 purchase totals = $554.08       $593.8363   $593.83   $593.85   $593.80