Only the funds that would go toward the investment process - aka those that won't be touched during it - should be used to invest in anything, not only in BTC.
That way, we will ensure that our needs are fulfilled, as well as the needs of the future us - with our actions.
That's why the ability to manage financial resources is needed because in living life, it's not just about investment.
If someone does not prepare reserve funds for other needs, the investment will not go well because under certain conditions when we need money, we have to sell some of the assets we have.
If market conditions are in decline, we will experience losses when selling these assets to meet our living needs.
Untouched funds can be used to buy in larger quantities when the price drops and that is an opportunity to accumulate bitcoins in much larger quantities.