We are talking about the Final Five immediately after the fall of Earth, two thousand years before they met the Colonial Cylons... They can't do shit. Sure they resurrected once, but the factories to build more bodies are on Earth and they've been nuked.
"When they need to" as an alternative to death or old age, because that's exactly what will happen if it takes longer than three days to get to Caprica from Earth if they didn't load their ship with the food, water and resources to make a transgalactic voyage if they have the storage bays to hold all that.
Sure they can keep trading through to new bodies to stay young and gorgeous while continuing to man the helm for a 30 year trip from planet A to planet B, but why would they want to?
They do have that projecting thing, but 30 years in a tiny box is boring and going to drive them spacecrazy.
Then there's the resources and power requirements to keep a skin Job awake walking around in that tiny box of a ship which increases the pay load which increases the fuel reserve, both of which probably increase the size of the engine which again increases the fuel reserve.
Unless you mean that they can force ageing process to reverse by "regeneration", I'm not sure what you mean that I do not mean at all. When I begun this I was saying "resurrection as an alternative to stasis" which began to become a bit of a mouthful very quickly.... They upload into a hard drive from a body that has nothing wrong with it, which is probably going to die immediately because there's no brain processing it's functions, then their mind "sleeps" in storage for 30 years, and then they download into fresh new bodies, when they have reached their destination after x amount of time.
No ftl, and we don't know enough about their relativistic engines to know how long a trip from Earth to Caprica would take, and even if it was just 2 weeks from the final fives perspective... That's a lot of food, air and water for 5 people unless they resurrect every time they get hungry, but really packing a couple boxes of corn flakes has to be easier than yet more skinjob replacement bodies to deal with all the many possibly terminal eventualities if they chose to stay awake rather than boxing themselves.