You might also check with the investor you assign the contract to. When someone assigns a contract to me, I like to use my own title company due to what I have negotiated, and because I typically pay all closing costs. I also like to have control of the transaction, and dealing with people at the title company I have a relationship with already makes it that much easier. That said, I am not opposed to using another title company if the rates are similar, especially if preliminary work has already been done, allowing the deal to be done in a shorter amount of time.
Bottom line, talk to your end buyer, do what makes sense for him or her. A good investor will take control of the transaction, make sure all goes smoothly, see that you get paid at closing, and look to you for more deals!