File off the tab? Do you mean the flat portion (or in some cases, the little nub) that sticks out on the interior portion of the spacer between the top race nut and the top locknut?. All that does is keep the top race mostly unaffected when you tighten the top lock nut on it and the top race. It keeps the top race from spinning too. Some steerer tubes have flats and even slots to accomodate these and some do not. Yes, you can file those down to make the spacer fit on the steerer tube, but you will have to invest on a second headset spanner to hold the top race in position as you tighten the locknut on it and the spacer if you do. Have the same thing going on with my Vitus Carbone resto. The vitus 979 fork on it does not have a slot or flat portion on the steerer tube threads, so I will have to file down the flat portion of the spacer on the new Stronglight A9 headset that I am installing on the frame or just reuse the plain spacer that came with the DA headset on the bike when I got it. Either way, I will still need two headset spanners to complete the job..

Chombi
84 Peugeot PSV
85(?)Vitus Plus Carbone 7