If I create a sqlserver db and don't enable use full text indexing, can I add it later? Currently using sqlserver 2005 but going to be upgrading to 2008.
Would there be disadvantages in terms of performance, storage space, etc in allowing for full text editing now although I wouldn't need it for a long time if ever?
Yes, you can add it later. I would recommend not implementing it until you need it, because of the additional administrative overhead. There is no real advantage to doing it before you need it, unless you can only configure the server once, as there are some additional permissions you need to make sure are in place.
I cannot add comments but...
In SQL Server 2008 all user-created datavases are full-text enabled by default
Though, it is just a possibility. One should also create full-text catalog and full-text indexes as well as to have installed and run SQL Full-text Filter Daemon Launcher