I think our error would be must not have dangles, which could be an exception (like at the end of mains where we have blow offs). We do have the rule for valves not being covered by a line but would not give that error in this scenario if still touching the endpoint of a line.
So after thinking about it more, the topology isn't an issue really, but if you want to do tracing in the geometric network or UN in the future, I don't see how the trace would stop at that valve if it's position was closed. It would just bypass that valve since it is not in line. I think it would be worth it to get your valve points in a position that works with the network now, otherwise you will have a lot of work later on.
We had a similar dilemma years ago and another idea that came up that we didn't go with was making a symbol with maybe an arrow on it and rotating it to point on what side of the main the nut was on. If you don't have that many butterfly valves, that may be something to consider.