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 $127.90   10.161%   $140.8959   $140.90   $140.90   $140.90  
  Purchase #2 $174.75   7.736%   $188.2687   $188.27   $188.25   $188.30  
  Purchase #3 $17.60   7.528%   $18.9249   $18.92   $18.90   $18.90  
  Purchase #4 $53.38   13.496%   $60.5842   $60.58   $60.60   $60.60  
  Purchase #5 $81.74   5.274%   $86.051   $86.05   $86.05   $86.10  
  Purchase #6 $27.99   11.716%   $31.2693   $31.27   $31.25   $31.30  
  Purchase #7 $112.76   8.136%   $121.9342   $121.93   $121.95   $121.90  
  7 purchase totals = $596.12       $647.9282   $647.92   $647.90   $648.00