Happily, I thought of this last week and ordered an extra quad of power tubes for my amp… and a full set for my preamp (I already had a full backup set for my CD player and amp). Monday I checked the site (The Tube Store) and they shut down their site to catch up on orders because of the onslaught. I received my tubes today, so I have full backups for all of my equipment and a few extra power tubes. so, good idea… but a bit late.
On the other hand, tubes are far more reliable than they are generally thought. I have the spare tubes I bought for about ever tube component I ever owned, but never used one. So, my recommendation is to relax. It will be OK. If when you have a problem, you will find a solution.