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 $25.42   9.407%   $27.8113   $27.81   $27.80   $27.80  
  Purchase #2 $36.77   5.500%   $38.7924   $38.79   $38.80   $38.80  
  Purchase #3 $63.41   7.383%   $68.0916   $68.09   $68.10   $68.10  
  Purchase #4 $155.60   9.404%   $170.2326   $170.23   $170.25   $170.20  
  Purchase #5 $130.46   11.287%   $145.185   $145.19   $145.20   $145.20  
  Purchase #6 $28.88   4.358%   $30.1386   $30.14   $30.15   $30.10  
  Purchase #7 $173.62   7.669%   $186.9349   $186.93   $186.95   $186.90  
  7 purchase totals = $614.16       $667.1864   $667.18   $667.25   $667.10