@sinbad - I think being proactive is always a good idea, but from what I have read it does sound like part of the routine of things you look for in an ultrasound, but why not ask? I think it is always good to err on the side of caution.
I do not know about the choroid plexus cysts being hereditary, but if you have some concerns about the cysts regarding Trisonomy 18 you will see a common theme with rare disorders - the word "may" in describing if it is a hereditary item or not.
From what I can tell some disorders can be traced and you can see a strong genetic link, but with others and especially rare disorders it is a little more difficult and as you can imagine with pregnancy and all the factors involved it is even more difficult to isolate causal factors.
I have a therapy client who has Trisonomy 18, and just to reinforce how rare this disorder is his parents have yet to meet another person in all the support groups and through doctors who have the same specific disorder.