I work with all mediums in Digital.
As far as mograph deliverables you have to be on top of your clients needs.
I always do the trickle down for most of my broadcast/film projects since they all seem to shoot at 4K DCI.
However, you have to keep other aspect ratios in mind such as Quad HD which equates well to HD for titling, credit rolls, mograph, etc…
Lower thirds I dont worry as far as resolution but I build everything in AE vectors (as much as I can) so that I can go up and down.
But if you build it at a decent size e.g. I do a ton of VFX and always told to keep that to 2K so at times I will do the mograph in that size and it seems to scale nicely up and of course down.
Its a deep subject but you will run into some funky ones such as digital signage.
Ive had to do some weird sizes when dealing with sports venues, entertainment installations, special events etc…
As Mads states, you need to be aware of it all.