Personally, I'm in favour of the "speed of plot" warp scale...
More precisely, if the story happens in a given sector, where players may have to travel between systems during the adventure (and speed being an issue), I try to map the sector in advance and define the distances between the various important points (in term of raw distance or just "time at warp X").
For any other travel, the time needed is the one fitting the story. Should the players notice a discrepancy between travel times from a story to another, there is always the explanation that the subspace condition changed, that a sun went nova and that they have to avoid the sector, that they need to resupply somewhere, and so on...
Hope this helps
"The main difference between Trekkies and Manchester United fans is that Trekkies never trashed a train carriage. So why are the Trekkies the social outcasts?"
Terry Pratchett