If You Aren't Even Sure, Others Might Wonder Too
The biggest danger in the situation you describe is that you yourself aren't really sure where the line is. Everyone else in your company will probably have an even less clear view of your relationship, so this is a big sign of danger.
The danger? If you are accused of misbehavior (whether it be negative towards the person you are in a relationship with, or 'positive' in form of favoring them compared to other people), how hard would it be to defend yourself?
If you are in a very unambiguous situation like being two rank and file team members, then it would be far less reasonable for anyone to think you have an inappropriate relationship which is violating the rules. But when you are maybe-kinda-sorta in a leadership position, such as unofficial assistant or informal tech lead and regularly report to your manager how other people are doing?
Well, that is enough to make a reasonable person wonder. Sure, maybe nothing is wrong, but if a reasonable person from an outside perspective can wonder, that puts you in a very bad position.
You Don't Want to Seem To Be Hiding Something
From your manager's perspective, especially if you are asked to report on other's performance and progress, if you are in a relationship that would cause you to have a "conflict of interest" that makes you less likely to report bad behavior, or more likely to give overly positive reports, or even that makes you less likely to give any report for fear of being seen as playing favorites...it sure would be nice to have some idea of what's going on so that they'd be able to factor that into your assessments.
And if the fact you are in a relationship just comes up and is a total surprise? That could get uncomfortable quickly, and they could begin to think back and wonder, "just exactly how long has this been going - and do I believe they've been giving me honest assessments in the past?"
Don't Ruin a Good Thing
It sounds like you are coming to be in a favored position in your team, and this suggests you would be in a good position in the future for promotion. A loss of trust could end that entirely. And as for your budding relationship - realizing you lost out of opportunities because of your significant other probably won't help kindle any flames.
If you are sufficiently unsure about whether or not this is something you should report, you should probably strongly consider asking your manager if they think you'd have anything to report. You could also consider talking with HR (preferably with some kind of trail/documentation), as reporting something they don't care about (they decide you aren't in a supervisory role so they don't care) is probably less risky than them finding out about something they believe you should have reported to them long ago.
This whole situation is a general gray area and is strongly dependent on your personal situation. In all gray areas it's usually better to get yourself out of the gray area by being cautious, rather than just standing around and waiting for someone else to decide just how dark that shade of gray really is.
