One of the items that gets in the way of using a 'customers' font is while it may be what 'they' want it may not do the job as far as directing traffic to their business. After Joe's advice on a sign design I did some surfin' and found a neat site that had some good lettering / size / color info.
If the sign is a sidewalk sign artistic lettering is very cool, if its a driveby sign then the rules for fonts change. Anyhow that is what I have been using as a guide. As Ted often says we are ALL in the sign business (so that MAY make us all EXPERTS)!
And yes this is just my opinion!