Now with the BTC price still sideways, I'd rather keep my spins as well.
On balance it makes more sense to use them up because of the interest they will give you meanwhile. I don't know exactly the yield in terms of spins you might get returned but even if it was only 1 or 2 spins per day extra you might as well take it I think. My guess is FBC never foresaw people on purpose saving spins, I do it myself sometimes but Im not feeling it presently.
You are not wrong in your thinking about getting interest on the savings.
However, the interest rate is 4% per annum on the savings. That is really a tiny portion of our savings. It is also beneficial only if we are holding a large amount of satoshi or whole bitcoin in savings mode. Then only we get better interest on our savings.
On the other hand, saving WOF has got two key points: 1) Not playing it right away when the Bitcoin price is not doing so great has dropped the rewards per spin also. 2) Spinning it together will increase the odds of hitting better rewards. Now if this is really a working methodology or not, the reward that is seen together is always going to be better.
Meanwhile, here is my progress in terms of WOF fillers:
