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 $6.66   4.108%   $6.9336   $6.93   $6.95   $6.90  
  Purchase #2 $16.27   5.360%   $17.1421   $17.14   $17.15   $17.10  
  Purchase #3 $13.39   13.605%   $15.2117   $15.21   $15.20   $15.20  
  Purchase #4 $227.59   13.404%   $258.0962   $258.10   $258.10   $258.10  
  Purchase #5 $85.61   10.422%   $94.5323   $94.53   $94.55   $94.50  
  Purchase #6 $27.57   3.854%   $28.6325   $28.63   $28.65   $28.60  
  Purchase #7 $14.79   9.789%   $16.2378   $16.24   $16.25   $16.20  
  7 purchase totals = $391.88       $436.7862   $436.78   $436.85   $436.60