Medication coupled with good work ethics starting from an early age can improve add. But if you stopped right after highschool then yeah, I would say that if you still are having add type trouble then it would be a good decision to go back on meds for as long as you need. Their are no longterm negative side affects for this type of medication.
I would go talk to your doctor. I am 30+ and was not diagnosed until I was 20+ and it is just as "Sandman2" said a lifelong thing. I would would not hesitate in talking to your doctor and getting set up for an evaluation. Good luck!
ADD is a life long thing. If you have the right job, medication is probably not needed. School is a whole different issue. Part of it depends on what you will be studying. If there is a lot of memory work or lectures, it might be very difficult without medication.
I would seek treatment now, so that you will have the choice of when to use or not use the medication. It maybe that you only need it for tests or studying. But, you want to have that choice when you need it, not after you have failed the test.
Also, if you had an IEP or a 504 in highschool that for example gave you more time on tests, etc. You can still use that where ever you go to school. If you didn't get any special help, then its worth seeing a counselor and making sure that you get what ever special help your doctor recommends. Also if you are in school more than part time, get their medical insurance. It will be cheap and help cover some of the costs involved.
Good Luck