well i have never seen a rear wheel bearing go bad on a 99+ mustang. but if you have high mileage it might not be a bad idea to replace the axle seals and bearings.
now i HAVE seen the axle seals go. but remember you can go back later and do the bearings and seals. all you have to do is remove the c clips and take the axles out, very easy.
i would say 100k miles do the axle seals and bearings if it makes you happy.
but stay the hell away from that gasket. the cover on our cars is sealed with rtv from the factory and there is a reason.
a little while back i had to rebuild a rear end that had 4.10's put in at another shop. it blew up because the fluid leaked out of that gasket.
after i was done with it i resealed the cover with rtv.