Alot of STD's have a six month incubation time.  you could be spreading the wealth for a good six months before you have any symptoms and then you have no way to figure out who gave it to you and who you might have given it to.

Best to be tested on a regular basis so if something is discovered, you know who you were with between the periods of being tested so that would narrow down where it came from and who you need to contact.