Nobody cares if it's web only. It could run at 12fps and still look good if there isn't much movement. Whatever works.
the web does not care. 30 fps will give you smoother motion but larger files than 24, Character animation works well at 12. I build all my projects so they are usable on multiple platforms so I stick with the standard frame rates for NTSC on everything except a few specialized projects.
I suggest you read through this info and specifically watch the last video: AE Basics
The only caveat I'd add is maybe to use the same frame rate as any video that might be included.