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 $138.75   6.737%   $148.0976   $148.10   $148.10   $148.10  
  Purchase #2 $19.25   6.503%   $20.5018   $20.50   $20.50   $20.50  
  Purchase #3 $77.93   4.410%   $81.3667   $81.37   $81.35   $81.40  
  Purchase #4 $135.63   7.651%   $146.0071   $146.01   $146.00   $146.00  
  Purchase #5 $120.36   5.412%   $126.8739   $126.87   $126.85   $126.90  
  Purchase #6 $123.36   5.865%   $130.5951   $130.60   $130.60   $130.60  
  Purchase #7 $25.89   12.438%   $29.1102   $29.11   $29.10   $29.10  
  7 purchase totals = $641.17       $682.5524   $682.56   $682.50   $682.60