While I'm sure it happens, I don't think that "evidence laundering" is particularly common, especially at the federal level. Cases I ran required an "initial notification" that succinctly described how our agents were notified of the potential criminal activity. The fear of having a case thrown out, or being turned down months into a high-level investigation because an attorney is uncomfortable with the likely outcome, is huge in ensuring a valid investigation is run.

Now, that's not to say that cops wouldn't do this in order to finally get a case around a particular subject who was able to sidestep previous investigations or something. I just doubt that it happens often enough to be worthwhile.