Baste Baste Baste!  You cannot baste enough!  Well, maybe you can.  Basting will allow you the luxury of checking placement of a sleeve or a dart before doing the final sewing.  Also, it makes life easier if that sleeve didn’t fit into the bodice perfectly, OR, in the case of almost every seamstress, you sewed the left sleeve into the right armcye! Uh Oh!!! Who hasn’t done that?  Seriously, when in doubt.  BASTE!