More specifically, does it matter on a PCB if routing differential pairs if they cross reference planes?
Do they need a reference plane?
Is there even a reference plane with a differential pair?
I don't do many high speed designs, when I have in the past, I make the routing as short as possible between high speed connectors and PHY's so I don't have to worry as much about matching, transmission line effects, ect. I have also used a continuous ground plane in all cases. But now I'm wondering...
My intuition tells me that a continuous ground plane in the case of a differential pair is not a big deal because there is little or no return current, so the only thing that matters is ensuring that the lines are matched.
A reason not to would be cross coupling of noise through capacitance into the differential pair, but noise will appear equally on both pairs (if the capacitance is the same on both pairs) and subtracted out.