My stance is that things like sending email should be explicit calls so they are obvious. Sending email when registering a new user, for example, should be in the if user.save branch in your controllers or, if you have a more SOAish app, in the service that creates users.

def create user = User.create(params[:user])

  if user.save
    Emailer.new_user_email.deliver
    render 'welcome'
  else
    render 'oh shit'
  end

Or, refactor that out a bit (ONLY IF NECESSARY!)

def create user = UserService.create_user_from(params[:user])

  if user
    render 'welcome'
  else 
    render 'oh shit'
  end