Are you talking about the brokerage statements that you have to send with the details of your stock transactions? It would help if you kept all your related questions in one thread, so that all the relevant information is in one place.
I think you can safely print the brokerage statements two-sided. For the tax return itself, if you are filing by mail, I always recommend printing on only one side. But for the brokerage statements, especially very large ones, I'm sure two-sided printing is okay.
