I shouldn't think it necessary to do all of them, just a good representative sample. You're hopefully not using bind variables so the same query with different values could yield different execution plans anyway.

commonly executed reports would be a good place to start, along with the longer running reports. You might be able to set tracing on at the instance level and capture most of them, but then if you have your front-end metadata in the same database then you'll get a load that you're not so interested in as well. Watch your disk space if you go down that route.