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 $17.83   12.750%   $20.1033   $20.10   $20.10   $20.10  
  Purchase #2 $75.45   11.464%   $84.0996   $84.10   $84.10   $84.10  
  Purchase #3 $19.41   5.807%   $20.5371   $20.54   $20.55   $20.50  
  Purchase #4 $166.81   10.247%   $183.903   $183.90   $183.90   $183.90  
  Purchase #5 $21.54   7.561%   $23.1686   $23.17   $23.15   $23.20  
  Purchase #6 $151.50   13.226%   $171.5374   $171.54   $171.55   $171.50  
  Purchase #7 $175.66   8.208%   $190.0782   $190.08   $190.10   $190.10  
  7 purchase totals = $628.20       $693.4272   $693.43   $693.45   $693.40