How to leave low voltage wiring behind wall?
The common practice for future expansion is to install the box and put a blank cover on it.

That eliminates the requirement of chopping into the drywall to find the wire. It also eliminates the need to create as-built documents and store them for future reference so you can find the wires later.

My recommendation is to install device boxes with ENT (referred to as "Smurf tube" since it is bright blue) connected to them from the basement or attic. Then when you decide to install a structured cabling system it can be the latest technology. Or, if you decide to update to fiber in the future you will have easy access to these boxes.

Communication / data is a dynamic field and planning for the future is difficult if you use a permanent wiring method stapled into the walls. In commercial buildings we normally install 4" square boxes with 3/4" conduit and single gang mud rings for the drywall. That way wiring can be easily replaced in the future. 

Good luck!