Sort of, but there are some tags you probably don't want to pollute your own top-level namespace with, like a, b, i, object, p, q, and s. There's at least one tag (var) which is not allowed as an identifier on its own.
What's more of a problem is if you have something like this: https://gist.github.com/2499913
The problem is that the "var p" in the for loop will be hoisted to the top of the scope, and so when you call p() in the first line, it will actually be undefined.