Where do you live? Do you live where a lot of salt is used in the winter? I am guessing yes since you are asking this question. If you live here in the south, I would not worry.

I would not recommend sandblasting the undercarriage, as the original paint/undercoating is usually very high quality. Sandblasting the undercarriage would expose bare metal and I do not think that any aftermarket undercoating would offer the protection it needs.

I would just recommend that if you do drive the car in the salt, to keep it washed under there as much as possible. I have heard of some northerns actually "painting on" used oil on the undercarriage.