Be careful doing this kind of thing. Some potential problems are:

In case of a crash your error handler has to bring the application back or
the user will loose other spreadsheets they may have been working on.

Users will believe that spreadsheets that they had open are gone and they
will try to reopen them. This will cause problems as they are already open in
the hidden instance.

If the user tries to open other spreadsheets they will open in the hidden
instance of Excel.

The user will open new instances of Excel and then when you re-activate the
original instance of Excel they will have two instances which can cuase some
difficulties.

I am not saying don't do it just be aware of the side effects of hiding the
application...