There is no need to do this preventative...when the gasket starts to seep, get it done...AT AN INDY SHOP!! The dealer will charge you $1000+ for this...you can find an indy shop to do it under $500. They leak because the bolts are breaking...DESIGN FLAW that BMW should have fixed!
I am going to caution those of you who, like myself, are amature DIY'rs. I have done various low level MX jobs on my E90 330i, such as thrust arm replacements, brake/rotor job, filters, spark plugs...etc.
Yesterday, I attempted the VCG change...and failed (I had to have it towed to a shop for them to finish it). There were several complications. First, there were two bolts that broke, hence the leaking. One was broken off down in the head. I didn't have the proper tool to remove it...it was lodged in there good and wouldn't come out without and extractor tool. Second, the oil separator hose that come out from under the Intake manifold and connects to the valve cover is VERY VERY brittle and broke when removing it. The challenge, at that point, is getting the other end disconnected from under the intake manifold...and the associated wire which connects WAY under there. Third, the wire connectors (you have to remove several) are like working with potatoe chips...they are so brittle that you cannot avoid breaking them when they come off. Lastly, torquing the bolts on the VGC without a digital torque wrench with an angular mode would be nearly impossible. The back bolts have little to no room, so getting an angular dial in there is a no go.
This one, in my opinion, is worth letting someone else do.