[DUG] Open / Close TTables & TQueries

[Charlie]

I'm using CodeGear 2007 with dBase tables.
On my order form which is actually on two tabsheets (one has fields 
which are for the order table and the second tabsheet has fields which 
are for the order detail table) I have several TDBLookupCombo controls. 
For the lookup to occur the lookup table has to be 'not closed'.
The 'order' form is called from the program's main form.
My question is should I open these tables when the order form is created 
or open these tables when either of the tabsheets are clicked on?
Secondly, when and how should these tables be closed?
Thank you,
Charlie