Since you have them I would give it a go, see what happens!
10.8 and 10.5 really aren't that far apart (especially considering that 10.6 was really just 10.5 stripped of PPC code), and I think most things should work.
Obviously File sharing and the like will work, and I would imagine the OS X calendar protocol didn't really change much so thats probably good as well. IMAP and POP email has been around forever so that's a go as well.
The only thing I could really think of as being in-compatible would be netboot. But even then, a 10.4 server can host 10.5 (maybe even 10.6 images) so even that might work (come to think of it when i was at the apple store they net-booted my iBook with a 10.5 image ... probably off a 10.7 machine[at the time]).