I'd say skip the first half of season five. Any episode with a long-haired blonde guy can safely be ignored. The second half, dealing with the fall of Centauri Prime and the fate of Londo Mollari, should be watched.
As for the first season, it's worth grinding through the occasional standalone author tract episodes to get the backstory necessary for the rest of the series. And if you find that a few of these are thinly-veiled remakes of ST:TOS episodes, you can take some solace in knowing that ST:TNG did the same...and that B5 often did them better.