Actually, it's not a good idea to put anything under car seats. The only time we (CPS Techs) put anything under a car seat is in the case of leather vehicle seats (a very thin rubber shelf liner), but now there is talk of even that not being a good idea.
A rolled towel or pool noodle can be used to help the recline of a rear facing car seat, but that's not under the entire base. It's placed just under the front foot of the base at the bite of the vehicle seat. A rolled towel or pool noodle should never be used with a forward facing car seat.
Full back boosters offer side impact protection and sleep support. Backless boosters don't. For these reasons, full back boosters are generally safer than backless boosters. Although I do use backless boosters when car pooling, etc. for convenience sake, my kids ride mainly in full back boosters. Well, DS (11) recently graduated out of a booster, but DD (9) still rides primarily in a full back booster. She's safer and more comfortable that way. That said, boosters that do have removable backs usually have weight limits before doing so, and Kaytee is correct that backless boosters must be used in a vehicle seating position that has a lap and shoulder belt and a head rest or high seat back.
While we're on the topic...
Kids are safest in boosters until at least 4'9" and can pass the 5 Step Test, even if they can legally ride without one.
http://www.carseat.org/Boosters/630.pdf
More info here:
http://www.parentingforums.org/local_links.php?catid=32
HTH
