"am wondering if I should be retroactively applying success flags to leads that have achieved that status previous to setting up the program? Is this advisable or should we only be looking forward?"
Richard, for revenue reporting purposes, I highly recommend you backdate the success statuses in the program for leads, since revenue attribution in Marketo is tied to program success.
Backdating acquisition date for the acquisition program and success date for program successes will ensure that reports showing first-touch and multi-touch revenue attribution are accurate.
Simply adding them to the program with a success status won't be enough in certain cases. For example, let's say I had a deal close last month, and today I create a program around a webinar which the primary contact for that deal attended. If I simply add the contact to the webinar today and make their status 'attended', the webinar program will not be credited with a portion of the revenue becasue the success date is still today (which is after the deal closed). If you backdate success and the success date is prior to the deal closing, it will be given revenue credit.
Anyways, something to think about...
Mike