In designing a crossover, I would think that to accurately build a crossover that 'crosses' over at a design frequency the parts tolerance needs to be up to spec to allow the crossover to work as designed...
What I don't know and am not an expert is how much variance means 'anything' or should we care? Is a filter circuit that has components off the center of their values really mean anything to us at all?
If a circuit designed to allow frequencies above 60HZ (example) how far off does a component have to be that would change this frequency???
I guess you would have to hook up some resistor and cap decade boxes, a signal generator and a Oscope to actually see whats happening...
I wonder if the crossover experts actually do this or is it all pure theory??
Again this is NOT meant to indicate subpar components in any speakers, but how much does this stuff really matter?
This topic may be as interesting as watching grass grow to some.... 
(at my age drinking beer and watching grass grow is somewhat enjoyable at times!!)
From my limited experience in DIY speaker building, I can repeat the "conventional wisdom" I've picked up from others concerning capacitors.
Many caps are sold at a rated value ±10%. For example, a cap rated at 10 µF, it might actually be anywhere from 9 to 11 µF. For most crossovers dealing with audio frequencies, this is fine. The individual woofers and tweeters sold by most manufacturers often vary more than this, and ±10% variations among drivers can sometimes make a much larger difference than ±10% in caps. It is comforting to know that the drivers that Jim uses from SEAS, Hiquophon, Aurum Cantus, etc. are among the best in the industry with regard to quality control and individual variation.
The cheapest non-polar electrolytic (NPE) caps available do work for crossovers. The rap against them has been that NPE caps dry out over time and their capacitance drifts out of spec. Exactly how long this takes is estimated at 15-30 years, and may not matter depending on your age and how long you keep the speakers. I'll probably be out of spec long before some NPE caps
. A bigger problem with NPE caps is that many never are within 10% of their rated value. Depending on who you buy them from, you might have to buy a carton full and measure each one to find several dozen that are the value you want.
For slightly more cost, you can buy metalized polypropylene (MPP) caps that seem to be made with much better tolerances and seem to keep their values indefinitely. For someone working on a small scale, it just makes more sense to use reasonably priced MPP caps instead of the cheaper but questionable NPE caps.
There are plenty of high-priced MPP caps available, but they do not seem to have better tolerances, say ±1% instead of ±10%. And finally, there are a variety of even more expensive caps made with different materials or construction. For example, there are film & foil caps some of which have silver conductor and oil or teflon insulation layers. Although there are those who swear by their superior "sound", it not at all clear just what is better or different about them, other than price. The debate on that continues.
