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 $240.98   8.392%   $261.203   $261.20   $261.20   $261.20  
  Purchase #2 $14.59   4.790%   $15.2889   $15.29   $15.30   $15.30  
  Purchase #3 $17.25   5.797%   $18.25   $18.25   $18.25   $18.30  
  Purchase #4 $165.42   6.833%   $176.7231   $176.72   $176.70   $176.70  
  Purchase #5 $191.44   7.119%   $205.0686   $205.07   $205.05   $205.10  
  Purchase #6 $129.94   3.386%   $134.3398   $134.34   $134.35   $134.30  
  Purchase #7 $82.64   4.256%   $86.1572   $86.16   $86.15   $86.20  
  7 purchase totals = $842.26       $897.0306   $897.03   $897.00   $897.10