Well, you might need a kilo, as you have the servos in the tail (mine were in the front, under the pilot's seat).
Personnaly, I don't like gliders that are too light, especially at the slope.
If you look at the video of the first flight, it's not very pretty, as it was a bit windy and the glider is all over the place in the turbulences! And impossible to land as it kept floating around!
That was much better with another kilo, but if you mainly do aerotowing, that's another story, but might be good to keep a ballast option for those windy days.