Headset Spacers, how many?

There's been some discussion regarding steer tube material and the number of spacers between the headset and the stem. All of the discussions I've read are focused on racing-quality lightweight stuff. If you have a thin aluminum or carbon fiber steer tube, it makes sense that zero spacers focus the stress right at the focal point of the structure. This is how a Pro racer, like George Hincapie in the recent Paris-Roubaix, can have a catastrophic failure. His steer tube snapped at mile 131.5 of the 161 mile race. This is a very demanding race, and this failure put him in the car with a damaged right shoulder. Big George'll understand it's a risk of racing, your customer will not be understanding about it if you set it up.
In the opposite case-by having a large number of spacers in place, the mechanic adds leverage to the rider handlebar input, which may also quickly fatigue the steer tube.

What do you do?

One industry expert has gone on record:
The number and placement of spacers does have an effect on the fatigue life of an aluminum steerer tube. Running less than 5mm of spacers under the stem can concentrate or "point load" a great deal of stress in one spot on the steerer. Therefore, Bontrager recommends a minimum of 5mm, and a maximum of 40mm of spacers between the stem and the headset to maximize the fatigue life of the steerer.
Scott Daubert Trek Bicycles

This has little to do with a $400 hardtail bike as set up by the manufacturer. But if you are replacing a fork, or even considering using a steer tube extender (BAD!), it is a factor to be aware of. If your customer is asking for a large amount of steer tube spacers, be sure to let them know they probably have the wrong size or type of frame. Do not install a large number of spacers, or get into a physics debate on the leverage difference of a long handlebar and tall stem vs. stem extenders etc. There is no set guideline, as it varies to the quality of the components.

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