Setting the spring height is the MOST critical portion of setting the valves up IMHO. You need a certain seat load with the springs and a certain 'open' load. Too much open load and you're wasting HP. Too little and the valve won't have enough load in it to keep the lifter following the lobe. Same with the seat load, too little and the valve may bounce off the seat; too much and you may damage the valve seat. Most cams come with a recommended spring load. Since the springs you've bought will fit many different heads, the springs have to be shimmed to achieve the spring loads recommended for the cam.
Have patience. It'll be worth it in the end.
BTW: The Comp Cams catalog has a wealth of information dealing with all aspects of the valve train. Real good reading. I'd suggest that everyone have a copy.
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Jeff Chambers
Trophy Stock #3
11.86 Seconds / 113.8 MPH
Chambers Racing Team