The way I look at it, which may be wrong from a mechanical standpoint, is:
(a) if the engine starts, I'm happy.
(b) my car is parked right next to the tractor in the garage and experiences the same temperature conditions, yet I don't do (and have never seen any one do) anything special to it other than stick the key in and start it (it's not a diesel).
Basically, if the diesel hasn't gelled, everything's good to go in my book!
My neighbor keeps all of his stuff outside - skidsteers, tractors, wheel loaders, etc - uncovered - he's never done anything special with them and they always start right up and work fine (may need to jump or change the battery though).
There's nothing wrong with using something, but I don't think it really adds much unless you really would otherwise have a problem starting it (ie: if your fuel did actually gel).