I've found that the line between "fanwank" and "continuity" is relative to the viewer. Everyone has a different idea of how much coincidence and crossover between series is tolerable and there's always going to be someone who's going to argue their case louder.
My personal rules -
A) One fact/event shouldn't contradict or attempt to eliminate another unless it's a third occurrence attempting to correct a mistake that was already made
B) Keep it timeline sensitive - an event in ENT shouldn't lead directly into an event in VOY without allowing for what could happen in the intervening years (not referencing a specific event here, just picked those series as examples as they're the farthest apart in the timeline)
C) It doesn't always have to be a full story. Sometimes a peripheral visual, audio or dialogue reference is enough to get the point across.