Distributing washers in various locations internationally would be statistically secure, but mathematically not. The probability of finding all washers would be less than winning the jackpot 100 times in a lifetime probably, but not zero.
I'll repeat what I said in another thread just yesterday:
I think we are bordering on the ridiculous here.
The chance of somebody accidentally stumbling across, say, 20 of your washers (assuming the remaining 4 words could be brute forced) which you have disposed of in various places around a city is infinitesimal. Spread those across a state or a country, and the chance is essentially zero.
If we are considering a targeted attack, then what's the likelihood that someone who wants to steal your coins is going to know enough about you to know your system, know you are disposing of it, and follow your every move for weeks or even months to pick up every washer that you throw away, versus some other far easier and more direct attack?
Still, this is wholly unnecessary. Just stamp over them, make them unreadable, and throw them in your trash. Job done.
And to fillippone, believe me or not I have not even thought about disposing of them in toilets or burying them in the ground, one washer every trip.
Putting a bunch of metal down your toilet sounds like a great way to get yourself an expensive plumbing bill.