The first question I would ask is:
"How much and who do you plan to string for?"
The title USRSA CS "Certified Stringer" means that you have passed a written and practical "hands on" set of tests designed to test your skill and knowledge of racquet stringing, repair and strings. It says that you have the knowledge and skill to do the job right!
If your customer base will continue to be friends and family, I would say it is up to you whether you obtain certification. Join the USRSA anyway, the books, stringers digest, etc contain alot of information and are the main course study material for USRSA exams. You can learn alot from these books....
On the other hand if you are thinking of getting a summer job at a local sporting good store, expanding your customer base beyond friends and family, or are looking for extra income through college, I think it would be a good idea. Certification is something you can use as proof of your abilities, and it might help you land a part time job in your local sporting good store while you are in school.
Good luck with your stringing.... and I hope this helps with your decision,
Geoff Jones
USRSA MRT