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 $25.72   4.525%   $26.8838   $26.88   $26.90   $26.90  
  Purchase #2 $17.42   3.673%   $18.0598   $18.06   $18.05   $18.10  
  Purchase #3 $125.89   7.442%   $135.2587   $135.26   $135.25   $135.30  
  Purchase #4 $71.41   12.410%   $80.272   $80.27   $80.25   $80.30  
  Purchase #5 $26.90   5.430%   $28.3607   $28.36   $28.35   $28.40  
  Purchase #6 $186.56   11.499%   $208.0125   $208.01   $208.00   $208.00  
  Purchase #7 $27.69   10.709%   $30.6553   $30.66   $30.65   $30.70  
  7 purchase totals = $481.59       $527.5028   $527.50   $527.45   $527.70