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 $222.66   12.288%   $250.0205   $250.02   $250.00   $250.00  
  Purchase #2 $123.73   8.289%   $133.986   $133.99   $134.00   $134.00  
  Purchase #3 $104.63   6.148%   $111.0627   $111.06   $111.05   $111.10  
  Purchase #4 $39.43   5.615%   $41.644   $41.64   $41.65   $41.60  
  Purchase #5 $26.41   13.852%   $30.0683   $30.07   $30.05   $30.10  
  Purchase #6 $166.44   6.827%   $177.8029   $177.80   $177.80   $177.80  
  Purchase #7 $215.56   12.376%   $242.2377   $242.24   $242.25   $242.20  
  7 purchase totals = $898.86       $986.8221   $986.82   $986.80   $986.80