I guess it depends on your situation, if you can ride the vol and still meet all your obligations that require fiat, then 100% bitcoin shouldn't be a problem for you and you can just get that one lump sum and swap out for cash when needed

If you don't have that, the cash buffer, be it 20%,30%,50% you'll have to do the math might come in handy since you cover your expenses and then the bitcoin is just for savings