I have been in the sign industry since 1974 and I have never worked in a shop that actually had a licensed electrician on the payroll. Few sign companies can afford the wages an electrician would demand anyway.
However, in many areas, including ours, sign installers are allowed to hook up lighted signs if the hookup is near the sign (within ten feet, I believe is the rule here). We are not allowed to run a line from the service panel, and we are not really supposed to install controls (timers, for example).
The electricians union here gives provisional membership cards to sign company employees who build or install signs. They cannot work as real electricians but they can work on signs. At one shop, even I was a member of the IBEW (International Brotherhood of Electrical Workers), though I was a painter. In fact, everybody out in the shop had a card. We produced UL listed signs, but the UL inspector never asked to see credentials. He only wanted to see how the signs were built.
Of course, rules may vary in other areas.
Most electric signs are incredibly simple in construction. Anybody can learn to wire them. Having said that, it has been my experience that many sign installers have limited ability to troubleshoot electrical problems when they occur. Typically, many will just change lamps or a ballast and walk away, believing they have solved the problem. I have seen as many as four ballasts installed in the same sign in less than two years without anyone questioning why the ballasts keep failing. I think this problem is fairly common.
When I was in business and I first acquired a bucket truck, with the intention of servicing lighted signs, I paid an electrician to show me how to use a meter and do basic troubleshooting safely. He was willing to do it because he didn't want to mess with sign repair anyway as it wasn't worth his time. It was the best money I ever spent.
Brad in Kansas City