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 $208.59   13.426%   $236.5953   $236.60   $236.60   $236.60  
  Purchase #2 $226.30   10.380%   $249.7899   $249.79   $249.80   $249.80  
  Purchase #3 $135.93   13.517%   $154.3037   $154.30   $154.30   $154.30  
  Purchase #4 $21.39   12.752%   $24.1177   $24.12   $24.10   $24.10  
  Purchase #5 $31.65   3.324%   $32.702   $32.70   $32.70   $32.70  
  Purchase #6 $225.87   13.281%   $255.8678   $255.87   $255.85   $255.90  
  Purchase #7 $46.97   6.488%   $50.0174   $50.02   $50.00   $50.00  
  7 purchase totals = $896.70       $1,003.3938   $1,003.40   $1,003.35   $1,003.40