Why would you want to grease your saddle rails?
I use carbon assembly paste for my carbon seat post clamp and carbon seat rails. The idea is to increase friction between the two components and reduce how tight you need to clamp them.
The last thing I would want to do is grease or use any other lubricant on my saddle rails, not only could it get on clothing and saddle bags but it could impact the clamping force required to stop the seat from moving.
The only thing which it might assist with is permitting easy removal of the saddle and preventing the clamp or seatpost from sticking to the rails, although this is not a problem I have ever experienced myself