I have always been told that when everything internally that can be moved HAS been moved to help balance the plane, go ahead and add the necessary weight as long as the model is not too heavy overall.
I have a Top Flite Elder 40 kit (I promise I will get to it someday) and the manual says that 12 oz. of nose weight had to be added to the prototype to get it to balance. WWI era planes tend to have short nose moments, so as long as the overall weight isn't excessive, I wouldn't worry about it.