Actually, because a sidedodge has frames where you're vulnerable, nothing is technically safe to plan on using after one...but nobody's THAT scared of getting hit, so meh. The above ARE the best options, but you run the risk of the opponent getting their shield out just in time.
Personally I prefer to use shield-dropped fairs. I'm not sure, but I believe the lag from having to take your shield down before dancing blade, d-tilt, jab, etc...*except dolphin slash* actually make f-air the safest option out of shield, if you don't feel like plummeting helplessly. With a f-air, you can also retreat VERY far while using it, unlike dancing blade.
Now that I think of it, I'm not sure of marth's jump speed, so it's entirely possible that dancing blade is actually faster than a shield-dropped f-air. meh. Dancing blade also moves up the moveset queue, so keep that in mind.
Basically...you have to drop your shield first to use anything besides dolphin slash or an aerial. So I like aerials out of shield, particularly f-air retreats, mixed in with a followup.