I think it's important to bring in new blood, but some breeders I know start out with 3-4 lines and don't bring in any after that, for a very long time anyway. Especially with Kilham, SDA and Sendai, etc. I've seen (heard of) Sendai (SV) and SDA wipe out several colonies. It's scary stuff.

In Rats or Mice, I breed for the genetics; disposition first and then whatever goal I'm trying to reach as far as looks. I currently have 5 different lines of Mice (3 show, 2 lines I picked up from feeder breeders with good genetics). With the Rats I have 9 different lines. I likely won't have to add any lines for a while, which is good because it always makes me nervous adding new rodents to my colony that I know is healthy and disease free. Even when you follow quarantine, treat for parasites, etc. You have the problem of indeed having one bad male (or female) messing up your genetics.

The best solution is to do what suzuki did and start out with several strains so if one is diseased you aren't years into it and have it take you out then. It's better to lose the colony when you first start. Which is what I do when I get back into rodent breeding. I make all the connections to get a colony started again and then gather up all of the lines I want and quarantine them separately. After quarantine time is up for the first set, I won't add anymore lines until the my original breeders/pets have died.