So here is the scenario Im looking for an answer to. Ive restarted a new game, played all the way through up until the point where I go kill Benny. What I plan to do now is go do LR. I would like to Nuke both the NCR and Caesar for the loot, then go directly to the Tops and kill Benny. Then when I leave the Tops is when the NCR and Caesars guy run up to me and tell me Caesar and NCR have forgiven all my past sins, and please come see them. If I do this, will it prevent NCR and Caesar assassins from attacking me randomly in the future?

No idea, but you could always make a save right at the end of LSR, bomb them both, go deal with Benny, get the forgiveness from CL and NCR then walk about for a while and see if you get any hit squads after you.

If you can get to Ulysses and kick his butt before you deal with Benny, past all those Marked Men and Deathclaws, let us know how that works out.

Yes, yes it will work exactly as you have stated. I have already done it twice, and you get the best of both worlds if you can forget the break in realism. It adds some extra farming areas.

Yeah, but for some reason, killing the irradiated Legionnaires in Dry Wells results in infamy gains, while killing irradiated NCR in Long 15 does not. So, you can't really farm Dry Wells after clearing your rep with the Legion. Well, you can, but then your infamy goes up again.

You do know there is a whole big world to explore and you can reach lvl cap before ever setting foot in new vegas.....

As long as you are vilified by those factions they will send assassins after you, once your rep is other than vilified the assassins will stop hunting you.

It worked fine. Nuked them both, went and killed Benny, spoke with the guys outside Tops. Neutral with both factions