One major purpose for applying grease is to prevent oxidation. Oxides are insulators, so when you get a layer of that between two parts of a connection the result is heat and voltage loss. Electricians have anti-oxidant greases designed for the job, like GB Ox-Guard, found in the electrical supplies at Home Depot or Lowes etc.
Connectors have a tin-lead coating which provides a good connection when the metal contacts are wiped against each other when inserted. A thin film of grease there can't hurt. But where you really need it is on the connections to bare metal... like where the ground lugs bolt to the frame. Clean both surfaces with a fine emery or crocus cloth abrasive so that it's shiny. But if the surface on a ring terminal is a dull silver color, that's the tin-lead coating and should not be ground off (go too far and you find copper). Apply enough grease to exclude air around the joint.
I relocated the R&R on my bike yesterday, and while it was all open I greased the high current connections to the battery, starter relay, and the two frame grounds I found near the battery. Hopefully I won't be into the electrical system again for a while!