I have read on some of the posts that you should not complete some missions and save them for farming. With that said does this mean the main storyline missions or the side quests? If you don't complete the main storyline missions how to you get to the other locations that occur later in the story to farm them? Is getting a "run" the only way to get there? If I finish the whole storyline does that gimp me from farming some of the better locations? Any input would be appreciated.
You can re-do missions whenever you want. But it's some quests that you want to keep, because once you turn them in, you can't get them back (with a few exceptions).