Two sides to this one. While working in a lab will help you with a better understanding of dental anatomy and tooth morphology, as well as an understanding of materials and shades, is it something that is imperative? No. Sure depending on the type of lab (crown and bridge vs a denture lab) it might help with some aspects of dental school, but the current trend in alot of dental schools is to not have students due their own lab work. Sure you'll do a couple of denture set up, and process a set of dentures or two for experience, or you may have to wax up a crown or occasionally do a full gold casting or stack some porcelain, but most of that is a 1 time thing for the understanding and experience of doing it.
In private practice, the almost all dentists will send out all their lab work to be done. Simply put, it takes the lab techs alot less time to do the work than most dentists can, and when it comes to dentists doing lab work at the end of a full day of patients verses going home and paying a rather small amount for someone else to do it, almost every dentist is in the car heading home. About as much lab work as I do on a regular basis, is I'll "tweak" a couple of teeth during an esthetic wax try in during denture fabrication. I can honestly say that the last time I did a full denture set up on my own was in dental school, the last time I waxed up a crown was also in dental school. Could I still do it, undoubtedly yes, would it take me 2 or 3 times longer than my lab techs, undoubtedly yes.
The big positive for working in a lab before dental school is that admissions committees would definately look favorable upon this!