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 $12.80   8.464%   $13.8834   $13.88   $13.90   $13.90  
  Purchase #2 $77.48   9.154%   $84.5725   $84.57   $84.55   $84.60  
  Purchase #3 $146.63   8.328%   $158.8413   $158.84   $158.85   $158.80  
  Purchase #4 $222.92   12.237%   $250.1987   $250.20   $250.20   $250.20  
  Purchase #5 $7.29   12.480%   $8.1998   $8.20   $8.20   $8.20  
  Purchase #6 $148.62   7.340%   $159.5287   $159.53   $159.55   $159.50  
  Purchase #7 $39.30   13.750%   $44.7038   $44.70   $44.70   $44.70  
  7 purchase totals = $655.04       $719.9282   $719.92   $719.95   $719.90