Yes, you want to watch the movies in a certain order such that spoilers are not released.
Here is what I would suggest in terms of how to watch B5
The Gathering Season 1 Season 2 Season 3 Season 4 Ep 1 - 8 TV Movie Thirdspace Season 4 Ep 9 - 22 TV Movie In the Beginning Season 5 EP 1 - 21 TV Movie River of Souls Season 5 Ep 22
Then if you want to start on Crusade TV Movie Call to Arms Crusade Season 1
Oh, thereabouts. Those of us who watched the initial runs of any of the first four seasons in the U.S. can tell you they ran in fits and starts - a block of new episode, a block of reruns, a block of new ones. But yes, the basic idea is that each season of the series corresponded to one calendar year in the lives of the characters. The first episode is usually set early in January, while the last episode is late in December of the same year, give or take a few days. (This approach also applies to the first 21 episodes of season 5. The final episode skips ahead a number of years to tell the final chapter of the story of Babylon 5 and what was wrought by the people who passed through it.)
Andy's order for the TV movies is a good one, though I'd nitpick and put A Call to Arms in its proper place with the B5 TV movies. (JMS said he regarded the film as the last B5 adventure, not as part of Crusade, which is one reason why only two of the Crusade regulars are featured in it.)
ACtA is not, as is sometimes assumed, the "pilot" for Crusade. Crusade didn't have a pilot. The deal was made to go to series based on B5 S5, the series bible and some sample scripts JMS had written. The network didn't ask for an actual pilot in the form of either a sample episode or TV movie. JMS just chose to lay in the back-story for Crusade in the last of the four new B5 TV movies that TNT had already contracted for at that time.
Chronologically the Crusade series also fits in before B5 episode 522. If they get around to releasing the unsold Legend of the Rangers pilot, that falls chronologically in between River of Souls and A Call to Arms. That was a pilot for a separate series and featured only a cameo appearance by one regular from the original series, and therefore wasn't included in the B5 TV movie set.
Oscar, none of the movies are necessary to understand the five-year arc. In the Beginning adds some background to that arc but, again, isn't required. Its worthwhile to watch but make sure you don't watch it until after the third season at a minimum and really after the fourth since that was when it was produced. Thirdspace and River of Souls are essentially standalone.
If you haven't bought or rented the movie set while you are in the middle of Season Four- no worries. Just keep on going with the main shows. Its not worth it to stop and wait for the movies.
Something I found interesting about when the show first aired was that they wouldn't show the final batch of episodes of a season until the fall. That meant that most people only had to wait a week to see the "season cliffhanger" resolved unlike all summer like most shows.
Right, the TV movies are definitely optional. Even The Gathering is more back-story than anything because the original plan of shooting the pilot and then going right into production on the series was dropped. By the time PTEN aired the pilot, looked at the ratings and gave the final approval to Warner Bros. nearly 8 months had gone, actors were either unavaiable or roles had been reconsidered* and various plot and production elements tweaked to the point where the series was very different from the TVM that had spawned it. While the events of TG are assumed to have happened, you don't need to have seen the movie to launch into the series - "Midnight on the Firing Line" does just as good a job introducing the characters and basic situation for S1.
(*Some changes came between shooting and final editing, like when JMS gave up his original idea of making Delenn male and electronically altering Mira Furlan's voice for the pilot and the first season, before unveiling a female Delenn early in S2. Once that decision was made they redesigned the Minbari makeup to allow male and female versions and no longer had to cast smaller, more lightly built men as male Minbari to "sell" the idea that Mira was a guy and that the Minbari were more visually androgynous than Humans. )
Yes, absolutely. Leave them all until the end. JMS and most of the cast approach the extras the way they would for a movie - and nobody expects anyone to listen to the commentary track before watching the feature. So they all tend to assume that everybody's seen the entire series already (probably several times.) The psychology of actors (and some studio personnel) in this respect is interesting. Most of these guys had been doing convention appearances for a modestly successful cult-SF show for five to ten years at the time they did the interviews and commentaries. Their natural assumption was that only die-hard fans would spend $60 to $100 a season to own these shows. They never seem to have thought that the sets - once owned - would be lent out to friends or that some people who were curious about the show but never got into it might take a chance on the discs. This is one reason the entire industry grossly underestimated the potential market for TV on DVD in the first place and had to be dragged kicking and screaming into doing full-season releases by the fans. (B5 being a specific instance of that, as anyone who participated in the WB live chats will remember.)
I seem to remember that Londo wants Quadrant 37 destroyed in ep 1 and then for the next 4 episodes we are getting clues that something bigger is at stake.