I wash at 50, I'm lucky to have a variable temp knob

but I would wash at 60 if I didn't as I think all undergarments should be washed at higher than 40 to kill any possible bacteria.
I do tumble for about 15 minutes most times too, just to soften up as lots of my nappies feel quite crunchy otherwise and I love how soft and fluffy they feel when they're tumbled

Whilst I have a large stash there are definitely nappies which get used lots more than others as I don't use them in a rotation where they all get used, but use what goes with Missy's outfits