A client posed this question today, and I thought I'd let my email response do double-duty by posting it here:
Yes, it is safe to send them the manuscript before you have a contract. Until the POD firm approves it, they can't offer you a contract. They need to be sure, first of all, that it is not racist or hate literature. And, they have to be sure the document format and layout is something they can work with.
As the publisher, they have a legal obligation to produce quality, so they have to see it before you sign the contract. Nobody in the publishing book business wants to claim ownership of our work. I always say its like a shoe salesman - chances are they don't want to steal your shoes while you're walking around the shop in theirs.