The custom is some places seems to be to say Shir Hamaalos before bentching for the seuda mafsekes on Erev Tisha B'Av. Although we normally say Shir Hamaalos for bentching whenever we don't say Tachanun - which would seem to include Erev Tisha B'av - is the common custom to say Al Naaros Bavel for Erev Tisha B'Av (which is the most appropriate time to say the Tehillim focused on the destruction) or Shir HaMaalos?