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 $23.43   13.531%   $26.6003   $26.60   $26.60   $26.60  
  Purchase #2 $143.60   4.362%   $149.8638   $149.86   $149.85   $149.90  
  Purchase #3 $78.63   7.727%   $84.7057   $84.71   $84.70   $84.70  
  Purchase #4 $126.73   13.540%   $143.8892   $143.89   $143.90   $143.90  
  Purchase #5 $62.85   10.444%   $69.4141   $69.41   $69.40   $69.40  
  Purchase #6 $163.25   7.107%   $174.8522   $174.85   $174.85   $174.90  
  Purchase #7 $176.56   7.662%   $190.088   $190.09   $190.10   $190.10  
  7 purchase totals = $775.05       $839.4133   $839.41   $839.40   $839.50