Skip assets:precompile only if the assets are updated.

smart_assets_precompile.rb

def do_if_updated(tag, *sources, &block)
  to   = (latest_revision rescue nil)
  from = (current_revision rescue nil)
  doit = if !from || !to || !fetch(:smart_assets_precompile, true)
    true
  else
    run_locally("#{source.local.log(from, to)} #{sources.join(' ')} | wc -l").to_i > 0
  end
  if doit
    block.call
  else
    logger.important "Skipping [#{tag}] because there was no change in #{sources.join(' ')} between #{from} and #{to}"
  end
end

namespace :assets do
  task :precompile, roles: :app do
    do_if_updated("assets precompile", "vendor/assets/", "app/assets/", "lib/assets/", "config") do
      run %Q{cd #{current_release} && #{rake} RAILS_ENV=#{rails_env} #{asset_env} assets:precompile}
    end
  end
end