Out of curiosity, what is the root problem you are actually trying to solve?
Leo's recommendation is safest and simplest, but it won't be real time. You could also do what you suggested (after submit) - but after submit would fail to handle Deletion/Void and CSV Imports/Updates without scripts enabled.
You could also do both...
Workflow doesn't handle line level details so well, so without scripts involved I don't think this is an option yet.
If you must implement a solution I'd go with just a scheduled script to avoid extra complexity and long term maintenance.