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 $134.56   3.104%   $138.7367   $138.74   $138.75   $138.70  
  Purchase #2 $20.30   12.527%   $22.843   $22.84   $22.85   $22.80  
  Purchase #3 $27.42   11.709%   $30.6306   $30.63   $30.65   $30.60  
  Purchase #4 $200.51   13.863%   $228.3067   $228.31   $228.30   $228.30  
  Purchase #5 $24.26   9.506%   $26.5662   $26.57   $26.55   $26.60  
  Purchase #6 $30.47   7.681%   $32.8104   $32.81   $32.80   $32.80  
  Purchase #7 $152.98   5.808%   $161.8651   $161.87   $161.85   $161.90  
  7 purchase totals = $590.50       $641.7587   $641.77   $641.75   $641.70