Before you upgrade you should deeply consider the question "why upgrade?"
Mac OS X 10.5 Leopard is a mature and stable operating system that has some very interesting and particular features that are not available in most of the more recent OS X releases, including the Classic Environment and a transparent emulation layer that both allow you to run (possibly expensive or irreplacible) software that ran on earlier Mac system software and fundamentally different hardware, respectively, before Apple switched to PPC, released the first versions of Mac OS X, and then switched to x86 based hardware. After all, what isn't broken cannot be fixed, and often "fixing" something that isn't broken will indeed lead to the discovery at the worst imaginable moments that essential functionality is broken or no longer exists.
The affirmative answer to the question should be along the lines of:
1) there are security concerns that you hope to resolve
2) there are software bugs that you hope to resolve
3) there are new features available and you wish to take advantage of them
There is nothing wrong with remaining at a version of a stable and mature operating system that is perhaps getting long in the tooth, and this is especially poignant with older hardware if after upgrading you discover that you have lost something that you needed. The temptation and immediate compulsion to click "update" or "upgrade" as soon as it is available or one receives notice of it is definitely not considered a "best practice," regardless of what most users are doing or even what the software or hardware vendor recommends that you do. Only you can decide, after careful consideration, what is in your best interests, as going backwards sometimes is not an option (as is the case with Apple's iOS operating system).