Go ahead and let me know if you already know a lot more about shims than I do and I'm giving you way too basic info. Right now I'm enveloped in a 'K' cam swap into my '83 245 with a B23F n/a. I've learned a good deal of information about shims in the short time I've been questioning the brickboard, so heres what I can tell you.

The base circles (the half of the teardrop on the cam lobe which is circular) are different from cam to cam. Because of this the valve shims, which are washers that space the base circle on the cam at a proper distance away from the lifter, must be correctly sized for the new cam. Your haynes or bentley manual should show you the proper clearance between the cam and the shim itself. It should also give you the formula for calculating the correct shim size based on current shim size, proper clearance, and the clearance you measured with the cam installed.

If you put shims in which allow too little clearance between the lifter and the camshaft lobe the valves won't stay closed as long as they should, and the exhaust valves won't dissipate heat well enough from contacting the cylinder head properly, and they will eventually have a hole burned through them from overheating. If you put shims allowing too much clearance between the lifter and cam, valve noise will be noticeably louder, and may accelerate valve train wear due to the cam slapping the lifter/valve mechanism instead of just gliding smoothly over the lifter.

Now if you're talking about properly shimming the cam based on ramp on the cam lobe or more complicated cam profiles, I can't help you. I hope what I've given you helps. Good luck.