I will be quoting a few statements from Jobst Brandt on page 71 in his book. David, take these quotes over to the Cat forum so others won't use Elso's crap advice!
"With tensioned wires as spokes, the wheel can support loads only to the point where it's spokes become loose. At this point the wheel will collapse. Therefore, for greatest strength, spokes must be as tight as the rim permits."
"In practice, however, spokes should be slightly looser than the maximum the rim can sustain, because at maximum tension, failure of a single spoke can severely deform the rim."
"It has often been suggested that looser spoking will improve the cushioning of a wheel, for instance one used on rough roads. Because the elasticity of spokes arises from the material properties of steel and is not affected by more or less tension, no change in ride quality can be achieved by loose spoking. Spoking with less than optimal tension only forfeits strength and durability."
I think that the last sentence in the last quote is the most important to remember as a rule of thumb.
Unfortunately, leaving it up to your lbs to take care of your wheels is a real crap-shoot! (actually, leaving ANY aspect of the care of your ride up to a lbs is a crap-shoot in most cases!) Very few shop mechanics have ever had any schooling in wheel maintenance or building, and it takes patience and a certain touch to get it right. Not everybody can do it.
One last little tid-bit that Jobst barely touches on is how badly a wheel will warp when just one spoke breaks. When Rafael's rear wheel broke a spoke last year, he knew it, because the rear end suddenly started wobbling enough to feel it. Before I tried to true it, it was a bit over 1/2" of-center near the broken spoke. It took a lot of work to get it back into true with one missing spoke, but we had no choice for the next day's ride. I got it true with one missing spoke, and picked up several new spokes the next day. I dismantled the whole wheel that night and put it all back together, spreading all of the new spokes evenly through the wheel just in case they had different properties than the rest. It trued up very nicely then, and is still rolling as I write this!
If a spoke breaks, simply sticking a new in place and tightening it won't fix the problem. Spokes around the broken one will pull the rim out of true beyond what a new spoke can restore. Surrounding spokes must be loosened and the whole area must be trued. Actually, it is best to loosen the whole wheel and start over with truing. Doing just the affected area can take longer, and certainly be much more frustrating, than starting over.
