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 $250.99   11.713%   $280.3885   $280.39   $280.40   $280.40  
  Purchase #2 $14.55   9.520%   $15.9352   $15.94   $15.95   $15.90  
  Purchase #3 $33.29   5.288%   $35.0504   $35.05   $35.05   $35.10  
  Purchase #4 $148.40   10.826%   $164.4658   $164.47   $164.45   $164.50  
  Purchase #5 $101.28   10.725%   $112.1423   $112.14   $112.15   $112.10  
  Purchase #6 $17.75   12.201%   $19.9157   $19.92   $19.90   $19.90  
  Purchase #7 $77.26   8.702%   $83.9832   $83.98   $84.00   $84.00  
  7 purchase totals = $643.52       $711.8811   $711.89   $711.90   $711.90