The more traction, the better. Customers are traction. Traction is anything that de-risks the investment and shows that customers actually like your product (even if it's not done) and you are on your way to product-market fit. If you don't have customers, then you can show things like early sign-ups or letters of intent. Always better to have customers. You don't even have to be making money directly from your product. Some companies consult first.