I'm getting ready to install a new gasket from Cummins on the timing case cover. Any opinions on whether I need to prep the gasket and case/cover with something or just clean everything well and put the gasket on dry?

 

I use Permatex #2 on just the block side mainly to hold the gasket in place.
Don't know if this is the "recommended" way to do but it's always worked for me.
A trick that makes this job go easier is to get a couple of studs or cut the heads off a couple bolts the same size as the ones that hold the cover on, screw them in loosely.
They will help keep the cover and gasket lined up while installing.

 

And use a rag and some brake cleaner to clean the surfaces real good before hand.