From a viewpoint of personal responsibility, both the severity of your fetish and the nature of the relationship determines how open you ought to be about it.
For a committed relationship, be that monogamous or open, you need to come clean about it and pretty soon into the relationship. Otherwise you could be with someone for 20 years before they discover that you gross them out and then you've all but destroyed their life: they might view those decades as a betrayal particularly since you knew all along.
The severity aspect alone comes into play with tricking. If you're into, say, animal sex, then--besides that I'd think you a real douche to begin with by my sensitivities towards protecting animals--you'd be an actual douchebag to risk passing whatever the fuck you can get from that onto any other human being, something I merely suspected but just googled and it's an actual thing. Ick fucking ick.https://en.wikipedia.org/wiki/Zoonosis
Oh look, it includes Ebola, what a treat.
So if you are a man, a man with balls, you need to own that or have it fixed if it prevents you from living your life honestly.
I experienced that on this very forum, when some creepy guy was making a play for me over the period of a year or two whatever the fuck it was. Besides that he didn't reveal himself as being a cruel bastard until he had manipulated my feelings toward him, but then he even, after all that time, first revealed that he likes to piss on people. Because being cruel wasn't enough, I suppose.
Pee on me and I'll knee your balls up into your throat. You'll not pee through your dick again but through a fucking catheter.
You need to be up front about your life if you want someone to be able to share your life.
People say they do to make themselves seem spontaneous but no one really likes surprises. Certainly not that one.