Profile for sure. Things that could be instant dealbreakers should be presented up front as much as possible.
This will protect you as much as it will protect the guys. A guy who has a couple of successful dates with you, and starts thinking you are really great, and then finds that out and it's a dealbreaker... they're going to be pissed.
And it will work in the reverse. You go on a few dates with a guy. Think he's really great. Start getting a big crush going on. And then you tell him, and he leaves. And then you feel like shit.
Weed that out first. Save him the annoyance. Save yourself the potential disappointment. Just be up front. Those that are down with it will still ask you out. Those that are not will be able to steer clear and avoid the inevitable let-down.
I've seen some profiles with that in them. But yeah I would tell them as soon as possible. Like when you feel the time is right depending on how the relationship is progressing. Not necessarily the first date but don't leave it too long either. If it were me I'd want to know this like right away because I think sex before marriage is important so we wouldn't be a match that way. But if you told me right away then I can decide if I'm ok with that or if we need to end things before we get into the relationship too far and feelings get hurt.