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 $146.58   6.787%   $156.5284   $156.53   $156.55   $156.50  
  Purchase #2 $152.92   13.498%   $173.5611   $173.56   $173.55   $173.60  
  Purchase #3 $15.92   11.589%   $17.765   $17.77   $17.75   $17.80  
  Purchase #4 $166.36   8.864%   $181.1062   $181.11   $181.10   $181.10  
  Purchase #5 $51.85   10.159%   $57.1174   $57.12   $57.10   $57.10  
  Purchase #6 $57.59   11.383%   $64.1455   $64.15   $64.15   $64.10  
  Purchase #7 $249.65   4.864%   $261.793   $261.79   $261.80   $261.80  
  7 purchase totals = $840.87       $912.0166   $912.03   $912.00   $912.00