Most of you decide probably decide who you will side with from the start. How early in the game do you actually commit and do quest in favor of that faction?

For example, if I want to do a legion playthrough if I'm with them from the start and automatically go against the ncr wouldn't most major areas will be automatically against me? If Im against the ncr early this would cut off access to most merchants and I would have to go chasing down traveling merchants in the wasteland. Wouldn't it be 10x more attacks on me by the ncr since they cover more land than it would be if I was against the legion?

It seems like siding too early is not a good idea.

If you're looking to do as many quests possible in the game, then siding early is a bad idea. And yes, the NCR far and away has the most quests and is in the largest number of areas in the game. You can actually play for a long time without choosing sides, though, as long as you're aware of the no return points in the main faction quest lines and don't cross them.

You can't receive quests from the NCR; and the NCR issues A LOT of quests. And no one issues bounty on NCR's officers heads like they do with the Fiends (I think there should have been an alternatives, but Obsidian got lazy)

Even worse: Ranger hit squads will come after you early on, and it svcks, because they will keep doing so until you're above Merciful Thug. Everyday. At least until you meet Mr House. Then if you continue then they will attack you everyday for the rest of the game. On the bright side, Ranger hit squads aren't as bad Legion Assassins.

Siding with a faction and disregarding the others is a bad idea soon in the game.

Generally I side with house, but I try to establish the highest possible degree of trust with all factions... which is possible... before I wipe out all resistance to House. It makes it easier to come and go as I please, and it paves the way for House and me than if I only go for one faction.

Generally I'll start with NCR due to a number of early quests (Mojave Outpost) involving helping them build order and you aren't introduced to the large scale imperialism.
Then after meeting the people at Westside, the Followers and the Kings I tend to be swayed more towards Independent.

It's really hard for me not to automatically side with NCR due to the number of opportunities they give you early on. You get a NCR rep real fast, while you almost immediately get bad rep with Legion as you have to kill a few in the NCR missions. Simple accessibility decides it for me.

And why is it, btw, that if you kill off a squad of Legion and bury their bodies in the NV desert, the rest of the Legion immediately knows about it? It would be nice if nothing happened to your rep when there are no witnesses.

And how do you even join the Legion? I'm only on my 2nd playthrough and it seems by the time you get a chance to hang out with the Legion you've already killed so many of them that they attack you on sight.

I tend to wait until a while after the Benny Vegas encounter before committing, though I usually make an early enemy of Legion.

The easiest way to do it is not to hack them off before reaching the Strip for the first time -

Even if you do get negative reputation with them before reaching the Strip,

Hah, thanks, I'll keep that in mind =)