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 $59.95   10.595%   $66.3017   $66.30   $66.30   $66.30  
  Purchase #2 $36.51   10.226%   $40.2435   $40.24   $40.25   $40.20  
  Purchase #3 $75.93   7.695%   $81.7728   $81.77   $81.75   $81.80  
  Purchase #4 $228.58   3.378%   $236.3014   $236.30   $236.30   $236.30  
  Purchase #5 $76.37   6.719%   $81.5013   $81.50   $81.50   $81.50  
  Purchase #6 $187.63   4.142%   $195.4016   $195.40   $195.40   $195.40  
  Purchase #7 $126.48   9.653%   $138.6891   $138.69   $138.70   $138.70  
  7 purchase totals = $791.45       $840.2114   $840.20   $840.20   $840.20