Every time I deploy my Rails 3.2 project to Heroku, rake assets:precompile is run:
$ git push heroku master
...
----> Preparing app for Rai
Sure! You'll need to create a manifest.yml in your_app/pubilc/assets directory.
The file can be blank. But ideally, you precompile everything locally, so deploys to Heroku would be much faster.
Make sure that you also committed the manifest.yml file when you're pushing to Heroku. Something like git add -f your_app/pubilc/assets/manifest.yml and a git push heroku master should suffice.