Tab is not nevessary. You should include a content for all your materials, the "Unit" of the materials is "document", document and document can not be stack together, but if one document contains more than one sheet of papers, they should be stacked together, otherwise the INS Officer may can not find the 2nd sheet. You can include a "Contents" sheet, put this content sheet at 3rd position (1st is I-140 and your payment, 2nd is cover leter), all documents put in the order of content list. That is all.
Content sheet sample:
Document 1. Form I-140 and money order (or bank check) with $xxx.
Document 2. Cover letter.
Document 3. This Contents.
Document 4. ******
and so on.