On my recent re-watch of Babylon 5
, I did hold off on "In the Beginning," for one, because I couldn't find my VHS tape of it and for another, I personally like watching it later on in the run. I personally don't agree with JMS as I think some things, such as the parts with Sheridan only work later in the series. Otherwise, you "meet" him and he becomes a focal point of the movie, but never appears again until Season 2.
Then there's the big mystery of Season 1, what happened to Sinclair on the Battle of the Line. "In the Beginning" spoils that.
I can see the argument for watching "In the Beginning" first on a re-watch. However, I don't think it's something you should watch before starting the series.