For new fields added to an existing table: Select Edit | Preferences and click the layout tab. You'll find a check box that controls whether new fields are automatically added to the current layout.
Don't know of a way to disable this for new tables.
However, that new layout is often useful as a "utility layout" when you need to establish a table context in a script. More and more of my regular data-entry and/or report layouts contain layout level script triggers and I am thus using Go to layout [ tablename (tablename) ] to specify a table for a find or other operation as it does not have these triggers and I thus avoid their possibly interfering with my script's correct execution.