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 $34.95   12.274%   $39.2398   $39.24   $39.25   $39.20  
  Purchase #2 $207.95   11.669%   $232.2157   $232.22   $232.20   $232.20  
  Purchase #3 $220.63   11.745%   $246.543   $246.54   $246.55   $246.50  
  Purchase #4 $91.46   3.860%   $94.9904   $94.99   $95.00   $95.00  
  Purchase #5 $17.49   12.357%   $19.6512   $19.65   $19.65   $19.70  
  Purchase #6 $84.38   11.421%   $94.017   $94.02   $94.00   $94.00  
  Purchase #7 $146.31   7.179%   $156.8136   $156.81   $156.80   $156.80  
  7 purchase totals = $803.17       $883.4707   $883.47   $883.45   $883.40