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 $248.94   10.869%   $275.9973   $276.00   $276.00   $276.00  
  Purchase #2 $201.61   3.297%   $208.2571   $208.26   $208.25   $208.30  
  Purchase #3 $78.94   3.382%   $81.6098   $81.61   $81.60   $81.60  
  Purchase #4 $149.51   3.719%   $155.0703   $155.07   $155.05   $155.10  
  Purchase #5 $72.82   9.682%   $79.8704   $79.87   $79.85   $79.90  
  Purchase #6 $136.86   11.773%   $152.9725   $152.97   $152.95   $153.00  
  Purchase #7 $134.32   4.197%   $139.9574   $139.96   $139.95   $140.00  
  7 purchase totals = $1,023.00       $1,093.7348   $1,093.74   $1,093.65   $1,093.90