Overriding capistrano assets:precompile task to precompile only when there are changes in any of the assets.

assets.rb

namespace :deploy do
  namespace :assets do
    task :precompile, :roles => :web, :except => { :no_release => true } do
      if capture("cd #{latest_release} && #{source.local.log(source.next_revision(current_revision))} vendor/assets/ lib/assets/ app/assets/ | wc -l").to_i > 0
        run "cd #{latest_release} && #{rake} RAILS_ENV=#{rails_env} #{asset_env} assets:precompile"
      else
        logger.info "No changes on assets. Skipping pre-compilation."
      end
    end
  end
end