It's rare to find a revolving credit line that you can actually know will be there when an emergency comes -- canceled cards, reduced limits, suddenly prohibited cash advances, exploitative rate changes, etc are all things that do happen during tighter economic times and just haven't been widespread in the last decade or so because money was cheap. Be careful not to be conditioned strictly on those freewheeling days that are now ending!
You can live the way you describe, and many of us have gone through periods of needing to rationalize it and do so, but with a clear head it's not something I would characterize as "the best blace for an emergency fund" because you just can't really trust it.
I thought it was only federal tax, not no federal tax?