Good plan! I used the Gleim products and love them.
You do need a signoff for the knowledge tests. The way to not "pay" an instructor is with the knowledge kits such as Gleim, King, Sporty's, Cessna, etc. and take their test (either CD/DVD, VHS, of just a paper test). Then you will send in your results to the specific school/publisher. They will return the required signoff for the written exam.
Now, before you solo an aircraft, you must take a written exam that is model specific, before an instructor can give you signoff to solo that aircraft.
The only written test you do not need a signoff for is one of the instructor and ATP writtens.
Brian
P.S. Several sites offer free online testing (which will be administered very similarly to your actual written exam), but do not give the signoff with these sites. Just makes for VERY good practice. MyWrittenExam.com is where I sent my students. Many others have them by now, I am sure.