Someone correct me if I'm wrong, but I think it may depend on where you would be crossing the runway. If you're closer to the 15R end of the runway, you'd be told to cross runway 15L and, vice versa, if you're closer to the 33R end you'd be told to cross 33R. I've also heard pilots being told to cross 15L/33R...
It should be whichever runway is ACTIVE
. If you're at the end of 15L/33R, and the 15s are being used, you should be told to cross runway 15L.
think about it this way. If the 15s are active, and ATC told you "XXX123, traffic holding in position, cross runway 33R", and you cross and look down to the approach end to runway 33R, chances are that you probably aren't going to see any traffic down there, because the 15s are being used for arrivals and departures.
They may use both names for the runway for clarity, but it shouldn't be because the pilot is closer to one end of the runway than the other.