Surprised no one has even mentioned the disadvantages of having a head start.
For example, the first guild to a boss can sometimes waste hours (or even days) on a bugged boss or an overtuned boss. These tend to get fixed, and there is some benefit to getting some early practice, but there's also waste when it comes to time that could've been spent doing splits or maybe another boss, or even just the extra fatigue that gets exerted on pulls that may become pointless after a hotfix. Sometimes Blizz comes out with literally game/strat-changing fixes or adjustments to fights that almost completely negate any progression you've had up until that point. In these cases, the time zone differences and head starts gets mostly negated because everyone's attempting a new version of the boss at the same time, even if others got to the harder/bugged first.
If you're a guild lagging being the leaders, you may get the benefit of getting to a boss that has been nerfed or hotfixed, and your guild likely has been following the front-runner's progress and strats... and considering that live streaming prog has been pretty open the last couple raids, it's really not that hard to see what your competition is doing, especially if they're in the lead. This doesn't mean that people just copy-and-paste what the front-runner is doing (that's more of a meta-slave mentality, which is usually done by non-top tier guilds), but it can be very informative about certain aspects of a fight and information that you may need. Simply put, having information from the front-runners mean you can spend less time/effort on your own prog of certain encounters, which means you may end up having more time/energy to burn once you catch up to the front-runners.
All in all, it's not really a black-and-white situation when it comes to world first with these sorts of things. I've perused the suggestions, and most wouldn't be good overall for the game or would just cause huge headaches. World-wide reset would probably be the best one could hope for, but as others have stated there's still inequalities. The world first title isn't even technically an official thing, and with most unofficial competitions it comes down to the community and the participants themselves to set the rules and guidelines. Just check out speedrunning for non-WoW games, as even the rules and guidelines can vary by region, but are generally agreed upon by the participating community.