I'm not sure, but I think it would be OK. 

I think that as far as preservation goes, it won't
prolong the life of the drawing though. The
sketches will be less likely to get bent and
stained in a frame, but the paper itself would
still deteriate (hopefully very slowly). Studios
are finacially motivated to use as cheap paper as
possible, so I doubt its acid free. I do know that
you can get the paper deacidified, but I don't
know how well that really works.