This is a very popular restaurant...I guess if I was sure I wanted to eat there, I would get reservations no matter when I was going. However, a lot of May is not all that busy, depending on when you're going. So what part of May will you be there, and what day of the week would you be going?
If it's Mother's Day weekend, or Memorial Day weekend, then definitely get reservations.
If it's the middle of the first week (like a Tuesday) you can probably walk-up and get seated within an hour.