i think you are talking about the axle seals. i personally would ask u to replace it other wise the leak would eventually drain ur transmission oil and ruin the transmission by running dry.
These seals are usually damaged when the mechanics are not careful enough in putting the axles back into the tranny once they have worked on the axles.
One more thing, if it is the axle seal. it wouldnt require opening the whole tranny. just pulling the axle out, putting new seal and putting the axle back in carefully. thats it.