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 $239.67   8.175%   $259.263   $259.26   $259.25   $259.30  
  Purchase #2 $34.64   6.630%   $36.9366   $36.94   $36.95   $36.90  
  Purchase #3 $59.94   7.448%   $64.4043   $64.40   $64.40   $64.40  
  Purchase #4 $196.63   7.595%   $211.564   $211.56   $211.55   $211.60  
  Purchase #5 $43.51   9.357%   $47.5812   $47.58   $47.60   $47.60  
  Purchase #6 $72.55   11.569%   $80.9433   $80.94   $80.95   $80.90  
  Purchase #7 $110.82   13.578%   $125.8671   $125.87   $125.85   $125.90  
  7 purchase totals = $757.76       $826.5595   $826.55   $826.55   $826.60