I, being the curious kind, did a little more looking into the order on various sites and came up with a more detailed suggested order list:
In the Beginning has the main action ocurring in the decade of the 2240's as a "tale" being told in the year 2278, but as mentiond previously if you haven't watched the series yet, you probably
should wait* till after you've watched most of the Seasons.
In the Beginning premiered between the
4th and
5th Seasons so that's one place to watch it. Others could be after most of
Season 5 and
River Of Souls but before the
last episode of
Season 5, or if you've watched it
all, you could start again in strict chronological order

. I've inserted
In the Beginning in
4 possible places to watch.
Thirdspace is supposed to occur between about
episodes 8 and
9 of
Season 4.
Then there are the spinnoffs/after the fact/many years discs:
The Legend Of The Rangers,
Calls To Arms/
Crusade, and
Lost Tales.
Crusade, being cancelled mid season, it's not complete (heh almost like watching old lost Dr. Who chronologies), and there are several episode orders proposed, aired, or otherwise (TNT, chronological, SciFi, and suggested by master chronologists). After much convolution looking at all these
Crusade lists, I distilled one with
one option.
(YEAR) Title broadcast date
(2240s) In the Beginning <---------*
(2257)
The Gathering february 93
(2258)
Season 1 jan - oct 94
(2259)
Season 2 nov 94 - nov 95
(2260)
Season 3 nov 95 - oct 96
(2261)
Season 4 ep. 1 "The Hour Of The Wolf - ep 8 "The Illusion Of Truth" nov 96 - feb 97
(2261)
Thirdspace "watch it after the 8th episode (or 2nd disc) of season 4". july 98
(2261)
Season 4 ep 9 "Atonement" - ep 22 "The Deconstruction Of Falling Stars" < - (last episode shows years 2361, 2761, 3261, 100226 ) feb 97 - oct 97
In the Beginning <--------- should not be watched first before Season 4 (or maybe 3) . jan 4, 98
(2262)
Season 5 ep 1 "No Compromises" - ep 20 "Objects at Rest" jan 21, 98 - nov 18, 98
(2263)
River Of Souls nov 8, 98
In the Beginning <--------- could also be watched just before the final episode of season 5, after "River Of Souls" and before the post-Babylon 5 "Call To Arms", etc diaspora:
(2265)
Legend Of The Rangers jan 02
(2266)
A Call To Arms jan 99
(2267)
Crusade: jun - sep 99
Code:
1 War Zone (8) B
8 Appearances and Other Deceits (13) B-G
9 Racing the Night (3) G
10 The Memory of War (2) G
11 The Needs of Earth (1) G
12 Visitors From Down the Street (4) G
13 Each Night I Dream of Home (5) G (Lochley intro)
6 Ruling From the Tomb (11) B <--------- (Lochley barely knows Gideon)
2 The Long Road (7) B
3 The Well of Forever (6) B
4 The Path of Sorrows (9) B
5 Patterns of the Soul (10) B
6 Ruling From the Tomb (11) B <---------
7 The Rules of the Game (12) B (Lochley and Gideon much closer)
(2271)
Lost Tales july 07
(2278) In the Beginning <---------
(2281)
Season 5 Last episode:
"Sleeping In Light" nov 25, 98
Your Parsecs Might Vary