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 $29.50   10.118%   $32.4848   $32.48   $32.50   $32.50  
  Purchase #2 $129.72   13.105%   $146.7198   $146.72   $146.70   $146.70  
  Purchase #3 $76.48   6.252%   $81.2615   $81.26   $81.25   $81.30  
  Purchase #4 $199.62   13.831%   $227.2294   $227.23   $227.25   $227.20  
  Purchase #5 $22.92   7.237%   $24.5787   $24.58   $24.60   $24.60  
  Purchase #6 $236.36   9.758%   $259.424   $259.42   $259.40   $259.40  
  Purchase #7 $58.78   4.713%   $61.5503   $61.55   $61.55   $61.60  
  7 purchase totals = $753.38       $833.2485   $833.24   $833.25   $833.30