Rather than merging cells to center headers, I prefer to have text centered
across selection. This avoids problems with deleting and filling columns that
are cosed by merged cells. The following sets up a command to toggle centering
across columns.
Open up the AppleScript Editor, paste the following code and save it as
/Users/<your-username>/Documents/Microsoft User Data/Excel Script Menu
Items/AlignCentered\sca.scpt.  The \sca in the filename creates a
keyboard shortcut control-shift-a.
-- Align selected cells across selection
-- Copyright under GPL by Mark Grimes
-- Saving with '\sca' in the filename creates Shortcut: Crtl+Shift+a
tell application "Microsoft Excel"
    --activate
    tell range (get address selection) of active sheet
        if (get count columns) > 1 or (get count rows) > 1 then
            if (get horizontal alignment) is horizontal align center across selection then
                set horizontal alignment to horizontal align general
            else
                set horizontal alignment to horizontal align center across selection
            end if
        else
            if (get horizontal alignment) is horizontal align center then
                set horizontal alignment to horizontal align general
            else
                set horizontal alignment to horizontal align center
            end if
        end if
    end tell
end tell
    
    
    
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