Replace your rear main if there is the slightest sign of it being old, leaking, whatever. Nothing pisses you off more than pulling a trans 3 months later to replace a seal you were just looking at.
If it is good and dry leave it alone though. Many of people dik up installs of front and rear mains. Don't be that guy.
I know you are asking about the transmission, but I would put a new rear seal in the engine while the trans is out. Since it is right there, wouldn't hurt and for piece of mind