IMO all three roles are of equal importance, thus you shouldn't just mainly focus on gearing say, tanks first. If the tanks aren't geared, you fail. If the healers aren't geared, you fail. If the DPS aren't geared, you won't make that enrage timer, thus you fail.
This is where loot systems come into play. Ideally marks and other items of high value should go to your raiders who are consistent in their attendance and have reliable outputs (just because Jimmy is there every week pulling his 2k DPS and dying to fires doesn't mean he should get it over Tommy who is also there every week and pulls 11k DPS and rarely dies to silly things).
All you can do is set yourself up to be the most fair you can be. Try not just gearing up one or two individuals but your whole raid equally...difficult I know, but it's more ideal than having just one OP player who may up and decide to leave one day for a better guild, leaving your raid a bit gimped.
In the end it's really just a pain in the butt balancing act of trying to gear up your raid body, ensuring that your guild progresses, and trying to keep folks as happy as possible. Gotta find what works best for your style
If you have a loot council you have to look at the encounter you are working on or those that will come after. Is there a boss with heavy group damage that has to be healed through - gear the healers, a hardhitter who rapes the tanks - gear the tanks, if youve got a boss with ridiculous amount of hp and a tight enrage timer - gear the dps. You cant answer the question without looking at the bosses you are facing.
If you dont have a loot council you dont have the option anyway for example.
It's generally a good idea to get your tanks and to a lesser extent, healers off to a good start. It depends on peoples gear levels when you start raiding though. Your tanks or healers might already have ok gear and them dying or keeping people alive isn't an issue. I guess as long as you can justify your decision you'll be ok.
Everyone will be in blues to start though. As with every single raid I've played for the last 5 years, you get ALL the gear you want if you show up each week and do good. People who complain about gear are usually the ones who show up once a week.