Depends on what you’re trying to do. If you’re trying to stack sats, it makes sense to spend the fiat. In which case, you might reasonably take out loans against a portion of your stack and short the fiat. Also, paying in fiat for Cashback in sats, such as with Fold.

If you want to spend from your Bitcoin directly, that’s harder but not impossible. It’s easy actually but “hard” if you factor in taxes and KYC. It’s best if someone you buy from simply accepts sats. Next best might be purchasing a gift card through Cake Wallet, Fold, Bit Refill etc. since it minimizes KYC and tax reporting to a certain extent.