You might as well track all uasset files.
git lfs track "*.uasset"
Why not all the files? LFS is an extension to git, mainly so you can put large files on github.com which prevents files over a certain size. Git is designed as a server-less VCS, so you can push and pull to a folder on your hard drive, your workmate’s computer, send bundles via email. These features don’t work as smoothly with LFS. If you use them you’ll have headaches with all the LFS tracked files. Practically however, most people don’t know about or use these features. They just use Git as a better alternative to SVN.
So if ever in the future you want to go to a different server other than github.com, you’ll have to do extra work with all the LFS files. Like download a different LFS client. Therefore you should try to limit LFS files to just the ones that could be big.
(Now that LFS has been out a while, I’m changing my opinion.)