Migrations add_index

Hi all,

If I have the following migration:

class CreateDocuments < ActiveRecord::Migration
  def self.up
    create_table :documents do |t|
      t.column :created_at, :datetime
      t.column :identifier, :string
      t.column :file_format, :string

    add_index :documents, :identifier, :unique => true

  def self.down
    remove_index :documents, :identifier
    drop_table :documents

Is it necessary (or recommended) to include the remove_index line in
the self.down method? I realize that dropping a table in MySQL will
automatically drop any indices on the table. However, I'm not sure if
this is the case for all databases. So would it be considered "good
style" to include the remove_index even though it's normally a bit