Yes, my default response to an unknown situation would be "no." But if I have time to consider it, I might decide it's worth the risk. Or I might just be feeling bored or mindful of the downside of my tendency to play it very safe (i.e. missed opportunities and mediocre experiences), and that might spur me into taking a bit of a plunge.
The wording of the question - "to say no to......" implies it's about situations where somebody else suggests or urges this step into the unknown. In that case, I'm less likely to do it than I am if I've thought of the idea myself. I don't know for sure whether that's because I'm prejudiced towards cynicism about other people's suggestions or whether it's just that other people's suggestions just aren't often very wise in terms of the way I function (I don't think they often know much about how I function, they mostly either assume I function in a normal or average way, or they project their own way of functioning onto me).
It's also usually better when I think of the idea myself because of the way I create and develop ideas - i.e. with my own nature very much in mind, either consciously or unconsciously, so if I have an idea at all it's kind of inherently matched to my nature. Like preparing a meal for myself, I'll usually create something much more suitable for my individual preferences than anything that somebody else would come up with, though exceptions are always possible because I don't always get everything perfectly right for me, and another person might happen to make a random change that I'd never have made, e.g. my wife made a salad recently which tasted great, and I think the reason is that she accidentally used sunflower oil instead of the "correct" thing (olive oil), and I'd never have done that because I usually work very carefully, defensively and narrowly, making very few mistakes, unless I'm in an experimental mood or just not functioning well. Similarly, other people's suggestions might be serendipitous like that.
It's an interesting topic, and touches on a matter I'm always returning to in my thoughts when I'm wondering how I might improve my ways. I don't think I'll ever embrace the unknown but I might do well to be a bit more friendly to it.