Yes, you don't want to waste money hauling around a bunch of extra weight with fuel costs high.

On the other hand, you don't want to have to make that "minimum fuel" call, or worse, not arrive at your destination!  In 1989 there was a tragic incident where an Avianca 707 had to hold for over an hour, missed the first approach at KJFK and crashed after losing power to all four engines due to fuel exhaustion while trying to return to the airport.

I don't know how many of us fly in an environment where we experience holds, unpredicted headwinds, etc. so we can often afford to be casual about our fuel planning, especially since we're not paying for it.  But I enjoy trying to make things fairly realistic, while yet keeping them as simple as possible.

Ideally, I suppose you would want to land with your legal minimum.  But if you don't have to hold, or there is a better tailwind than expected, you will be well over that if you have planned properly.

I have been using a cost index of 50 since there are good tailwinds going east across the Atlantic and I can get away with something low.  The 767-300 goes up to 9999.  I have not yet determined what practical effect changing the cost index makes.  It will be nice when the "insufficient fuel" message bug is straightened out.

I am uncertain to what extent real world CIs translate to FSX for this airplane due to vagaries of FSX weather, etc.  I don't think you could go wrong with 100.  This is a good area for frequent 767 drivers to put our findings together and come up with these answers.