No, you don't have to. Reserving is optional. It's worth doing anyway, because it marks the item as taken so two people don't end up buying the same gift.
Reserving and buying are separate steps. You can go straight to the store, but then nobody else knows you've got that wish.
Why reserving helps
Once you reserve a wish, a second person can't reserve the same item until you cancel, which is what signals "I've got this one." It costs you nothing, because a reservation doesn't charge you or place an order. It's just a marker. See "Does Wishpicks process payments?" And the surprise holds: by default the owner doesn't even see that anything was reserved, and your name stays hidden. See "Can I see who reserved or bought a gift?" in Gift Reservation.
Good to know
Reserving is not buying. A reservation signals your intent, and you still order the item yourself at the store. See "Can I buy a gift directly on Wishpicks?"
Each item can be reserved by one person at a time. If a wish is already reserved, someone else has it, so pick a different one.
Whether you need an account depends on the owner's settings. Some wishlists let anyone reserve without signing in, some ask you to sign in first, and on some, reserving is switched off. See "How do I reserve a gift before buying?"
Buying without reserving is fine too. Just know that someone else might buy the same item.
A reservation doesn't complete the wish. After you buy, only the owner can mark the item as received. See "What should I do after buying the gift?"