Here's the deal. (yes the engine cases are aluminum.)
First off the stock Phillips heads are steel, so the statement "steel v aluminum" is off the mark. the steel screws will stick to the aluminum, but its tolerable and takes a long time for issues. The stick occurs under the head where it contacts the covers, where the elements can get to it and not (so much) at the thread level.
Now I'm assuming rather than Hex Bolts, you actually got stainless steel allen screws?
What he's really talking about is the issue of STAINLess steel against aluminum. There is a chemical reaction that occurs, again over time, and wil stick the stainless to the aluminum worse than non-stainless.
You know we all use these stainless screw kits so there must be more to the story. And that is, you should use a readily available "anti-seize" compound when installing stainless screw kits. A little on the threads, and a little under each head. As so:
http://www.sacskyranch.com/antiseize.htmAvailable at all fine auto stores.
Having said that, I've put in many stainless kits way back before I knew about anti seize and lived to tell the tale. No big deal.