The wife and I are letting our teaching licenses expire.
Most backup plans we have that involve teaching would be overseas, where having a current teaching license isn't a necessity.
We decided that the chance we'll need to get down to our 13th backup plan or whatever (teaching in the US) is small enough, odds-wise, that the requirements to keep it up (small monetarily, for the license, medium monetarily for the continuing education classes, big time-wise in terms of the classes) were not worth it.
That's how I'd recommend calculating it... what backup plan number is going back to work? Is it plan A? Or plan Q? And is it required for all types of work you'd be doing (e.g. can you consult without it), and how long until you can decide you won't need that as a backup anymore? And how likely is it you'll get to that backup plan (e.g. if you're already at a 2% WR, extremely unlikely, if you're at a 4.5% WR, maybe more likely).
Turn it into a math problem. Yay! :)