If the enemy team has all ported back themselves, it's generally a good idea to port back yourself, yes. You wouldn't, though, if your AD carry does and you don't need to (full health and mana, but low gold) in which case you can farm. Also, it's sometimes an oppurtunity to gank mid (but not always).
Porting back at low health is generally a good idea if you're afraid they can kill you. However, if you can heal this is generally not necessary. And it's usually easy enough to stay hidden or safe. (I assume you play a champion that can't heal though.)
If you need to port back when your wards are almost gone, simply put another one before you go. You may not get the chance to go back later.
The best time to push a tower is when your lane is fed and you need to help another lane. Don't push if you're underfed.
And while you didn't mention it, bein low on mana with no way to restore it is a pretty darn good reason to go back too.
I tend to go back when we've pushed up to their lane, especially after killing one of them but not the other, and the one there (or even if it's both of them) is playing very safe (and I don't want to turret dive). Basically, if I have nothing to do OR if I'm out of mana OR if I need to buy wards are the main reasons I go back (and "having nothing to do" could be because one of the enemy went back or is dead).