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 $234.99   12.405%   $264.1405   $264.14   $264.15   $264.10  
  Purchase #2 $214.52   8.410%   $232.5611   $232.56   $232.55   $232.60  
  Purchase #3 $176.58   6.765%   $188.5256   $188.53   $188.55   $188.50  
  Purchase #4 $45.28   10.228%   $49.9112   $49.91   $49.90   $49.90  
  Purchase #5 $62.77   12.647%   $70.7085   $70.71   $70.70   $70.70  
  Purchase #6 $20.83   8.724%   $22.6472   $22.65   $22.65   $22.60  
  Purchase #7 $43.47   10.334%   $47.9622   $47.96   $47.95   $48.00  
  7 purchase totals = $798.44       $876.4563   $876.46   $876.45   $876.40