I wrote an article about shims because there are so many stupid things said about shim.
In German it's a nice title: "Wie schlimm sind shim?" (how bad are shim?) http://161589.homepagemodules.de/t33...sind-Shim.html
Leo Fender "invented" the mass-built E-bass with removable neck.
But Fender also had shims as replacement parts in its part list!!!
There have ever been shims in "the world of Fender" and no one cares as long as nobody knows - because nobody hears!
I even heard ofexcellent sounding Telecasters with metal washers as shim ...
If your bass sound good and feels good without a shim, it will sound good and feels good with a shim! A piece of wood veneer might be best. But Fender used a kind of "paperboard" too! I used real paperboard - and it works well!
There's no need to worry!
If you hear a differnece, it might not be the shim but your worry! Remember that people sometimes hear what they want/intend to hear!