Personally you can't have enough in your library- variety etc. But I always like to get my own when possible. In between projects, I always go out and record for the fun of it and being in South Africa, there are so many things that need to be captured that don't exist on present day packaged libraries like SIs or BBC etc. I have always had the philosophy that it is a sound designers obligation to build their own library as their career progresses. They are yours, unique to you and make you r projects sound fresh. There is nothing worse than relying on the canonized libraries and finding out that SI 6021-03 New York Traffic with horns ST has been used in hundreds of other projects and is easily recognizable.
That's my 2 cents.
Upfront sounds deserve more attention than a low volume background sounds. I argue that you are doing your project a disservice if you are spend hours on the background dog bark that could be spent on the fight scene that is happening at the same time.
In a perfect world we all would have great location rigs and access to all the explosion, animals, and vehicles we need to record our own. I generally dont have the time or budget to be too dogmatic about it.