I think the original question has been answered (yes, it's best to symlink your interface folder for ease of maintenance), but this has caught my eye:

I do simlink lot of folders :
Data, Account & Interface
If you symlink all this lot (especially the data folder!) then what's the point of multiple installations at all? Surely you're driving all the I/O traffic back to a single physical disk in any case?

It seems to me that symlinking in this way completely defeats the purpose of having multiple installations in the first place.

I'm obviously missing something - what is it?