It's up to you, there is no dictate on what you should have.
10.5 is getting pretty old. You would know that 10.8 just came out, so inevitably our software slips into obsolescence. However when upgrading there are gains and losses; gains being new features, losses being some loss of old applications. For example in 10.7 one loses the ability to run old PowerPC applications through emulation, but in 10.6 you still can do this for the most part, but ultimately depends on the individual application.
I think 10.6 is a good blend of old and new. But it is also obsolete now