You can file a copyright registration on the complete collections of all of your blogs/podcasts, but the registration will not cover any later added material. You have to file a new registration to cover the new material. Also, since “fair use” and damages analyses examine the proportion of the work taken, if you ever sue someone, you would probably rather have ten small registrations, than one big registration.
I usually recommend registration on a monthly, quarterly or annual basis, depending upon:
1)The value of the blog/podcast content;
2)The likelihood of infringement; and
3)How much you want to spend on copyright registrations.
For companies like Microsoft, that have employees blog for them, these “works for hire” may qualify for serial registration. As determining whether something is a “work for hire” is a complex legal issue, if you are interested in pursuing the serial registration, it is best to consult a copyright attorney to advise you not only with regard to registration, but as to initial ownership and transfer as well.