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 $23.53   13.514%   $26.7098   $26.71   $26.70   $26.70  
  Purchase #2 $142.34   13.152%   $161.0606   $161.06   $161.05   $161.10  
  Purchase #3 $48.66   9.542%   $53.3031   $53.30   $53.30   $53.30  
  Purchase #4 $105.37   8.635%   $114.4687   $114.47   $114.45   $114.50  
  Purchase #5 $222.46   7.807%   $239.8275   $239.83   $239.85   $239.80  
  Purchase #6 $217.95   7.705%   $234.743   $234.74   $234.75   $234.70  
  Purchase #7 $40.44   5.308%   $42.5866   $42.59   $42.60   $42.60  
  7 purchase totals = $800.75       $872.6993   $872.70   $872.70   $872.70