Is optimizing JavaScript for loops really necessary?

I read that it is advised to optimize loops in JavaScript by not reading the length attribute of an array every iteration in the loop header.

So, we should rather do this:

var names = ['George','Ringo','Paul','John'];
for(var i=0,j=names.length;i<j;i++){// Read array length once and assign it to a variable
    doSomeThingWith(names[i]);
}

instead of this:

var names = ['George','Ringo','Paul','John'];
for(var i=0;i<names.length;i++){
    doSomeThingWith(names[i]);
}

However, I created a small testcase to compare the two techniques, but sometimes the first case was faster and sometimes the second one.

Which version would you recommend?