For example, I have a named scope sfw_only in my Image model that returns images with nsfw == false.
I'm also using acts_as_taggable_on_steroids - and I'm trying to think of the most efficient way to do something like the following.
if !params[:tag].nil?
  if nsfw_mode
    @images = Image.find_tagged_with(params[:tag])
  else
    @images = Image.find_tagged_with(params[:tag])
    ... remove images with nsfw == true
  end 
else
  if nsfw_mode
    @images = Image.all
  else
    @images = Image.sfw_only
end