Do you have to keep some fantasies secret in order not to scare a dom?

Subs sometimes have scary or extreme fantasies. Do you find that some of yours are 'too far' to the point where it can scare a partner away? Or is the typical dom (male or female) open minded enough that you can tell her whatever you like? Assuming she can be trusted to play safe.

In particular I have a recurrent non-con castration fantasy. Is this something common enough in male subs that it's not a worry? Or is it extreme enough to put off a lot of doms? Obviously I wouldn't use it to introduce myself, but after knowing someone for a bit and being able to talk about fetishes.