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 $246.68   13.164%   $279.153   $279.15   $279.15   $279.20  
  Purchase #2 $145.54   13.136%   $164.6581   $164.66   $164.65   $164.70  
  Purchase #3 $123.76   8.504%   $134.2846   $134.28   $134.30   $134.30  
  Purchase #4 $199.99   8.554%   $217.0971   $217.10   $217.10   $217.10  
  Purchase #5 $131.71   9.334%   $144.0038   $144.00   $144.00   $144.00  
  Purchase #6 $59.37   5.486%   $62.627   $62.63   $62.65   $62.60  
  Purchase #7 $238.81   7.841%   $257.5351   $257.54   $257.55   $257.50  
  7 purchase totals = $1,145.86       $1,259.3587   $1,259.36   $1,259.40   $1,259.40