consider flashes dictionary lookup time vs checking yourself.
this code that looks through all objects within a movieclip
and calls the function render() for each movieclip;
Code:
for (i in mc) {
 if (typeof(eval(i)) == "movieClip")
   mc[i].render();
}
the code would cause flash to lookup the dictionary twice for each movieclip and once for other objects (the condition). i can code the same thing, omitting the check like this;
Code:
for (i in mc) 
   mc[i].render();
this will have the same effect as the first code fragment however now there is only one dictionary lookup per object. when flash discovers that the object is not a movieclip, it just does nothing.