I've been kinda pondering this because of the pally (78.6 and leveling!), and a post over at You Yank It, You Tank It made me decide to write about it.
I likely have a bit of a odd perspective here. After all my first toon has the exact same gear for main and offspecs (though I do keep meaning to get her some tier 5). And as for Lyllea, well, disc wants int and spellpower, thinks spirit is icky, and doesn't care much about mp5. Most people see spirit or mp5 on cloth and go "healy gear" so I've had more than one argument about what should be main spec gear for her. Mostly with caster dps, who are grumpy that the healer's rolling on gear they perceive to be theirs. My philosophy on that is "it's only reserved for caster dps if it has to hit, otherwise it's fair game."
I'm of two minds on the "queue as what you're rolling on" idea. On the one hand, I do agree that the tank or the healer should get first dibs on tank or healer loot. In a guild run or a raid, I would never roll on tank or healer loot unless I was performing one of those rolls or none of the tanks or healers wanted it. But in a 5 man, the rules are a little bit fuzzier, simply because I cannot count on my fellow puggers to observe those principles. More than once during Lyllea's gearing process I lost nice pieces of healing loot because a tank or dps was building a heals set. Didn't matter to them that I was the one actually healing their butt, all they cared about was getting themselves gear.
Rolling need on offspec gear almost seems like rolling need on the Frozen Orbs, simply a way to prevent yourself from being screwed. After all, if I go into a dungeon with Adowa as holy, and want to get ret gear, it's going to be a bit of a struggle. Loads of competition for dps plate and weapons, and there's sure to be some very annoyed plate dps if I started rolling on "their" stuff. But I could wait in the dps queue, and get my pick of spellpower plate (not much competition there). And then we have the time investment. The healing queue's quick enough that I could probably run twice as many heroics as holy, getting twice as many emblems and twice as many shots at gear.
I don't really think there's a clear answer to the main/offspec thing in 5 mans. I'm thinking that I'll probably do more or less what I do with shadow gear on Lyllea. I watch to see if the caster dps roll need on the shiny before I roll. If they need, I greed. Same thing should work for plate stuff, especially if I announce at the beginning of the run that I'm trying to gear my ret set as well as my holy set but will pass if a plate dps needs. Will I get screwed? Probably, but at least this way I feel like I'm being fair.
And in case you were wondering, I'm currently the lootmaster for my guild and our loot rules for raids are...
1. Main spec before off spec, main spec being the spec you're in the raid for. If we ask you to switch, you can pick which spec you want to be your main.
2. Once you win something you go to the back of the line
3. Hunters get preference over melee for ranged weaponry, melee get preference over hunters for melee weapons.
4. If you want something that isn't your armor type (like a caster druid wanting cloth), it's an offspec roll.
5. Patterns are open roll if you have the profession at a high enough level to learn the pattern on the toon you're on.
And the most important rule... Don't be a dick!
We pug a lot so they're pretty simple, but they work.