I perform forensics as one of my duties, and insist that anyone performing forensics has formal training. I think this is important for a few reasons; if you have to take the stand in a trial, it adds credibility and differentiates you from those that just poke around in the space, also, even though we all talk about trials and such, most of your work in a corporate setting will be HR type investigations. My feeling is that since what you do, and how you do it, can have very serious impact on a person's life, the least a company can do is make sure the personnel performing the investigations are properly trained.

Another random thought...
From the evidence protection perspective, make sure you not only take evidence spoliation into account, but also your company's records retention policies. You always want to keep information (evidence or otherwise) for as long as it is needed but not a minute longer.