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 $238.86   13.146%   $270.2605   $270.26   $270.25   $270.30  
  Purchase #2 $128.85   12.338%   $144.7475   $144.75   $144.75   $144.70  
  Purchase #3 $55.71   5.147%   $58.5774   $58.58   $58.60   $58.60  
  Purchase #4 $130.66   8.359%   $141.5819   $141.58   $141.60   $141.60  
  Purchase #5 $179.66   10.817%   $199.0938   $199.09   $199.10   $199.10  
  Purchase #6 $210.26   3.690%   $218.0186   $218.02   $218.00   $218.00  
  Purchase #7 $175.31   4.347%   $182.9307   $182.93   $182.95   $182.90  
  7 purchase totals = $1,119.31       $1,215.2104   $1,215.21   $1,215.25   $1,215.20