You mean runtime package? Not sure about it, to be honest, as I haven't used VS'08 yet. I guess you do need '08 package to run programs compiled in '08, and if you have that, you shouldn't need '05 package. But you might actually need both...
To clarify - I'm sure that if you have '08 package, you don't need '05 package to run what was compiled in '08. For program compiled in '05, I would guess that '08 package would also work, but you might actually need '05 package specifically to run those.