I think they'd be wisest to cast people who generally aren't associated with space opera shows. Would nuBSG's Adama have been as interesting had he been played by someone we'd already seen doing the space commander thing? Would nuDoctor Who have been as compelling with Patrick Stewart in the lead?
On the other hand, Bill Mumy and Walter Koenig were both excellent in Babylon 5. If their first appearance got you thinking about the kid from Lost in Space and Chekov, pretty soon you were just seeing them as Lennier and Bester.
I really think the important thing is casting actors that work in a role. Whether or not they've also done previous sci-fi work - it's gravy if they have, and not a big deal if they haven't. Stargate may have been susceptible to stunt casting, but if it was perhaps a better written show would anyone really mind?
And yes, to semi-echo
Bob the Skutter, Homeland is excellent and well worth seeing.