The 7th Game takes on more importance, regardless of who started serving, because there's often only one other "serving" opportunity left to break or be broken (to win the set cleanly).
Opponent served first
Going into the 7th game, you're tied at 3-all...with your opponent serving. If YOU break...it's now 4-3...with you serving to go up 5-3...and only one game away from the set. If you don't break, you're "down" 4-3 (even though you're technically "on serve")...and if your opponent breaks you in the next game...he goes up 5-3 and can serve out the set.
You served first
Going into the 7th game, you're tied at 3-all...with you serving. If you are broken (yikes), you're now down 4-3...and your opponent serves to go up 5-3, just one game away from the set. If you're not broken, you go into the changeover up 4-3...and hungry to break your opponent so you can go up 5-3 to serve out the set.
I think it's about the mental attitude of being "up" or "down"...and realizing the set's end is closer than you think. From a mental perspective and pure logistical standpoint, an early break has more of a chance to be recovered simply because there are/will be more opportunities and you think you're "still in it." By the time things are 4-3...there's only one more chance.
I do think there's something to that it takes several of service games to get a good feel for the game so that at the "midpoint" of the set (as Ron referred to) is probably a so-called momentum shift...on way or the other.