Roms are Roms are Roms.

As long as it works, play and enjoy!

Anyone can download the ROM code from http://www.planetarypinball.com/mm5/Williams/tech/roms.html Shops charging for ROMs are charging for the EPROM chip and time to erase/program the software to the chip. Many people burn their own chips or have friends who have burners.

Not now, no.
Back when games came from the factory, yeah, WMS had their own labels.

Yes, there are official roms with foil stickers that go on the Roms.

All other Bally Williams Roms that are sold without this sticker are technically bootlegs. But it is not enforced, and you can burn your own Roms as the licence holder allows the files to be available to download.

Officially-released Bally & Williams ROM software revisions that are Genuine Bally/Williams Replacement Parts and are supplied with permission of Planetary Pinball Supply, Inc.These ROM revisions include a $2 non-refundable charge which covers Williams Electronics Games licensing fee and a processing fee. Official terms of software use and licensing can be reviewed here. Your purchase and use of official Bally/Williams software on ROM chips that we provide indicates your acceptance of, and compliance with, the terms in the Williams Software License.

http://www.actionpinball.com/weg-swlicense.htm

http://www.thatpinballplace.com/ also sells licensed roms.

Pinball center sells official ROMs in Europe. The stickers Williams used in the past do not exist any longer and any ROMs with those stickers are new old stock. All ROMs are programmed into a generic EPROM. Official ones are additionally licensed from the copyright holder such as PPS for Williams. Larger distributors such as pinball center would not get away with selling unlicensed ROMs. Therefore, pinball center is your European source for official ROMs.

As ROMs can deteriorate over time, I would trust a new official ROM that has been freshly programmed and contents verified (part of the programming process) over a 20 year old ROM with a Williams sticker that may or may not still be good.

Sometimes early games in a run, straight from Williams' factory, had hand written labels.

Like Markmon said, get freshly burned ones and don't give it a second though where they came from.

I would certainly be wary of bit rot if buying NOS ROMs with official Williams labels.