In SAP, you need to choose either Plant Level or Company code level to valuate your material stocks. Once you've set, it is not possible to switch the valuation level from plant to company code or vice-versa unless data conversion (material master's accounting data) is carried out again.
We recommend to setting material valuation at Plant level because you can have separation set of account determination and stock type at each plant.
Also it is important that stocks must be valuated at Plant level,
when u implement the PP module the default valuation level will be PLANT.