Next backup after updating to Snow Leopard
 

I've just updated the OS from 10.5.8 to 10.6.3. For my next backup to, can I simply do a smart update or should I do an erase and copy?

Given the large changes between major OS versions, an erase-then-copy is probably a good idea (it'll be faster).

Thanks Dave. For minor updates in the future, say from 10.6.3 to 10.6.4, would a smart update suffice?

Absolutely.