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 $133.38   4.252%   $139.0513   $139.05   $139.05   $139.10  
  Purchase #2 $77.87   3.218%   $80.3759   $80.38   $80.40   $80.40  
  Purchase #3 $7.63   13.427%   $8.6545   $8.65   $8.65   $8.70  
  Purchase #4 $57.69   6.394%   $61.3787   $61.38   $61.40   $61.40  
  Purchase #5 $21.25   4.312%   $22.1663   $22.17   $22.15   $22.20  
  Purchase #6 $209.37   8.349%   $226.8503   $226.85   $226.85   $226.90  
  Purchase #7 $67.62   6.306%   $71.8841   $71.88   $71.90   $71.90  
  7 purchase totals = $574.81       $610.3611   $610.36   $610.40   $610.60