Yesterday Gaia Clary published an article on Machinimatrix.org about the state of mesh uploading. There are still ‘gotchas’ in the process, some of which have been fixed in the SL Development Viewer.
If you are uploading rigged mesh, it is worth your time to read the short article. See: Upload issues with rigged meshes.
If your rigged meshes are uploading OK with the current release viewer (3.3.4-262321), you are avoiding the things that Gaia mentions as problems. That is a good thing. Knowing about them will hopefully assure it stays that way.
Once the next version of the release viewer comes out rigged mesh uploading should be a bit easier.