I assume you are staining cells grown on slides or something similar??? If your antibody is targeted towards an antigen expressed on the inside of the cell (for example the c-terminus of a protein that spans the lipid bilayer) then you would need to permeabilize - especially if your antibody is a monoclonal antibody. Do you know anything about your antibody? Is it polyclonal, monoclonal? If in doubt why not try it on positive control samples with and without permeabilization.