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 $121.28   8.320%   $131.3705   $131.37   $131.35   $131.40  
  Purchase #2 $26.49   12.108%   $29.6974   $29.70   $29.70   $29.70  
  Purchase #3 $161.40   6.320%   $171.6005   $171.60   $171.60   $171.60  
  Purchase #4 $12.70   7.499%   $13.6524   $13.65   $13.65   $13.70  
  Purchase #5 $26.40   7.705%   $28.4341   $28.43   $28.45   $28.40  
  Purchase #6 $115.83   12.368%   $130.1559   $130.16   $130.15   $130.20  
  Purchase #7 $188.99   13.844%   $215.1538   $215.15   $215.15   $215.20  
  7 purchase totals = $653.09       $720.0646   $720.06   $720.05   $720.20