Depends what you are tagging, but if I were tagging releases, I would tag multiple projects in a release with this schema: PROJECT_NAME_ABBREV-DATE-TIME
So some example:
ASD-2013.01.12-1251
WWW-2013.01.12-1511
BJN-2013.02.22-0124
Doing this will allow you to have some nice functionality of tags:
git tag | grep PROJECT_NAME_ABBREV
You can tell how when a tag was made by looking at it, and you can split tags up for searching, etc.
The Assembla pull down will allow you to search and filter these tags easily as well.
Your tags will also be readable and manageable.
(Assembla does allow you to have multiple git repos in 1 space, however that is not what you asked, I would prefer to segregate each project into a different repo)