That's wholly up to you, depending on what your play style is at 80. If you have a particular faction you want to get Exalted with, you may want to do some of those dailies every day. If you like PvP, you might want to try out the ones on the river in Grizzly Hills. Personally, the only dailies I repeat every day are the Kalu'ak ones so I can get the fishing pole more quickly, and the ones that happen to be in the zone I'm questing in already.
For example, I'm level 79 and I'm questing in Icecrown. There are a bunch of dailies there, and I'll stop and do some of them because they're good XP and my other quests are right next to them. I don't go back to the Grizzly Hills (again, example) and do the PvP ones though, because they're kind of out of my way and I don't really like to PvP.
So it pays off to do certain dailies every day if they help you further your current goal. I wouldn't spend a massive amount of your play time each day running around Northrend completing every daily you can before level 80 just for the sake of doing dailies. It's not terribly efficient. Or fun, for that matter. Get through to the good stuff, don't worry about dailies too much. Go ahead and do them once the first time around just to see the quest, but don't necessarily do it every day.
Carang of Kargath asked...
My question is about the STV fishing contest. If someone turns in the 40 fish to win, can you still fish for the rare fish to turn in?
Yep! The fishing contest has a set start time, and a set end time. You'll always be able to fish up Tastyfish and the rare fishes within that time frame. The first person with 40 Tastyfish wins the main prize, but everyone can keep turning in the rares for the special stuff after that, right up until the Tastyfish schools go away.
How many guilds have already beaten Naxx on 10 and 25?
I don't have solid numbers for either of those things because I'm not Blizzard, but I know that on my server Naxx-10 was cleared two days ago in a few hours by a mish-mash of people, whatever level 80s the raid leader could find and knew personally. We don't have enough people to hit Naxx-25 yet. Again, that's on my server.
I do know Naxx-25 is being cleared on other servers, and it only takes 25 people who are level 80 and can figure out how to not get slimed by Heigan, and let's be honest, if you can't dodge Heigan's slime you need to practice how to use your run key.
Just because Nihilum and SK were the first group to clear Naxxramas doesn't mean it takes Nihilum and SK to clear it. It just means they were the first group with enough organized 80s to do it. Utterly trivial.