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 $86.92   13.715%   $98.8411   $98.84   $98.85   $98.80  
  Purchase #2 $149.87   10.139%   $165.0653   $165.07   $165.05   $165.10  
  Purchase #3 $108.94   12.358%   $122.4028   $122.40   $122.40   $122.40  
  Purchase #4 $81.33   8.287%   $88.0698   $88.07   $88.05   $88.10  
  Purchase #5 $176.57   5.716%   $186.6627   $186.66   $186.65   $186.70  
  Purchase #6 $161.48   13.267%   $182.9036   $182.90   $182.90   $182.90  
  Purchase #7 $21.63   12.573%   $24.3495   $24.35   $24.35   $24.30  
  7 purchase totals = $786.74       $868.2948   $868.29   $868.25   $868.30