I have never had to inspect a neon sign, so someone else better qualified may correct me on this from the 1990 NEC.
Article 600-31 (a) requires RMC, IMC, RNC, FMC, LFMC, EMT, or MC. An exception allows LFNC.
Article 600-5 covers grounding, but does not really address the conduits specifically. The exception states that Isolated noncurrent carrying metal parts shall be permitted to be bonded ... and grounded in accordance with article 250. This could be interpreted as requiring the grounding.
600-37(e) does require grounding of the attached metal parts according to article 250 for portable gas tube signs for show windows and interior use.
250-42 requires fixed equipments exposed metal parts likely to become energized ((f) over 150 volts) to be grounded.
250-75 requires bonding if they are to serve as grounding conductors with or without supplementary conductors.
250-76 over 250 volts requires electric continuity for metal raceways and cables.
I would require it based on this. I hope that this helps.
Jes: You brought a good subject to light. Over the years, here in NJ, neon has been a stepchild for the most part. (For contractors, electricians, and inspectors) We do service work for a national chain, and the neon we have seen is amazing. It seems the GTO was either "bare", in "plastic "hose"; or in greenfield. A plethora of connectors; and basically "anything goes"
The above all pertains to neon wiring that is "out of site"; above drop ceilings, behind storefronts (soffits)and within the "troffs" that channel letters are mounted to.
Lately; the AHJ's (of which I also am one) have been looking a lot closer at neon, and it is a tough subject to understand, Code wise. The sign contractors all say "we've been doing it like this for years" or that's how we always do it"
To keep this as short as possible, there have been quite a few fires, and I see quite a few potential shock hazzards. Neon window signs, hung inside; bare GTO thru the ceiling tiles, thru the same hole, transformers "sitting" on the ceiling grid; extension cords for 120volt power. The electrode terminations/terminals are "bare" allowing anyone to touch them. Newer signs have a piece of clear plexi to prevent the "touch"; but it still appears that no one knows howe to install the wiring.
To directly answer your ??; GTO is supposed to be installed within grounded conduit (flex/greenfield" with approved connectors at each end. Neon guys say the GTO bleeds to the Flex, and causes a short. That's why they leave it bare. I'm off to read the '02 NEC on the subject; NJ still is under the '99 though. John
PS: Sorry for the long winded post; if it's offensive to anyone, please let me know