as for the original question:
When i play single player, i only "savescum" on truely stupid mistakes costing me the game. If i was having fun i see no reason to say, "ok, picking up that shard was what the AI waited for, especially since right afterwards spire civilian leaders protested to bring the AI Progress Level up to MK3 and the AI used this for a CPA that will hit when the Shard is half way home, well, humanity is doomed".
I accept a certain back and forth, i tend to have a fortress system in a choke, and all beyond that is readily accepted to "fall", as well as golems or other stuff. But if it's game-costing, i see no reason to "stop" if i know i could do better.
As for multiplayer, i am currently playing with a friend new to the game...so yeah, we save-scum often. We play in "less-tactical"-mode, glassing the galaxy, against a lower-level bully and a higher-level assassin...so each of us "holds" one front, but as he's learning still, some reloads are just a necessity to prevent frustration.(i could always recover for him, but losing 4 systems, his whole fleet and some trader-purchased stuff is only "fun" the first time it happens...subsequent losses just seem to keep him out of the game forcing him to rebuild while i expand on my front AND clear systems on his for him to colonize...