Shimming a Guitar Neck Pocket
The neck joint on any guitar is the most important join, it's got a lot to live up structurally and tonally. If this joint is not 100% you're missing out big time.
Often I take a neck off a Strat or a Tele and there's random materials loose in there, business cards, sanding paper, a match stick or even cut up plectrums. The reason they've been put in there is to adjust the angle of the neck to help the set up out.
Nice try but by lessening the contact area between the body and neck you're affecting you guitar's sound
Here's How
Neck with poorly made shim - Bad coupling and air gap

Neck with tapered shim - Maximum coupling and no air gap

My Neck Shims
The idea is simple adjust the angle whilst keeping the most contact possible between neck pocket and the neck heel. I use maple veneer which tapers (gets thinner at one end). Each neck shim is made for the guitar by tracing round the neck heel shape.
So there you have it, A tapered hardwood shim made especially for your guitar or bass that will really improve your neck joint quality.
The next step...
For the ultimate bolt on neck upgrade I recommend my Stainless steel neck reinforcement bolts! >Actually BOLT ON your Guitar's Neck
Fender Micro-Tilt
Fender introduced the Micro Tilt design to allow easy adjustment of the neck angle without the use of shims. Thing is though, when you're neck is set at the right angle, when are you going to re-adjust it again? It's not like every month. Seems a little over engineered to me plus it still allows a huge air gap. I like to add a shim into these as well to fill up the air gaps and it really works. Honestly, bolt on neck joints need all the help they can get