For a time, it was common to raise a seed round without revenue or customers. Today, the short answer is still no. Lots of companies still raise seed funding without revenue. Having revenue and customers will certainly help you raise money; it will also help you raise it faster.

It’s OK if your team is awesome and has an exit or two under its belt already, or if you are playing in a market like social networking that doesn’t have early revenue. But if you’re not, then you need a big market, a great product, founder-market fit – and lots of market validation – to raise money. Customers and revenue take away the risk from investors and increase their likelihood of committing capital to you. 

Typically, you need to be building your product for one to two years, launch and generate some traction before an investor will commit money. If you've already launched and are seeing initial traction, it's much easier for investors to commit money. While not having revenue or customers at the seed stage isn't critical, having them is the ultimate market validation and significantly boosts investor confidence.

Remember, not all revenue is equal – investors respond better to steady, substantial growth than sporadic, one-off revenue numbers. Seek out market validation and ‘golf-ball’ traction that will impress the investor, not simply revenue.