Joe b.,
I think a lot of people are missing part of your statement. You say that your spirit level sight and ANY OTHERS you've tried you can't keep them centered up. Not to be offensive about it but you might have a problem. OR, the quality of the sights you have access to aren't the best. It might help us to know what make sights you are talking about? All the advice you've been given so far is exactly right.... for a quality sight, and if a quality sight the maker will stand behind it.
MI's comment about the thickness of the liquid is right on. A friend had this "centering" problem and got a vial with a thicker liquid and it helped a great deal. I think he got it from MVA, but not certain. MVA also has different colors of liquid which makes a tremendous difference in being able to see the bubble.
To answer your questions in order.....
1. Yes you should be able to hold the bubble centered off the bench, cross sticks, and offhand, altho it's not the easiest thing to do offhand.
2. Yes. To shoot accurately you do need to be able to hold the bubble in the center, or at the least in the same position for each shot. Center just happens to be the easiest. Considering the trajectory of Schuetzen or Shilouette rifles a slight cant either way can throw the bullet off quite a bit either left or right. In Schuetzen we're trying to hold 3/4 MOA at either 100 or 200 yds. If you have a gun that will consistently shoot 1/2 MOA then you only have 1/4 MOA to play with. Someone with better math skills than I have could probably figure out how many degrees off you would need to be to put the shot outside the 25 ring. It isn't much.
What range is a spirit level useful?.... Of course a lot would depend on what kind of accuracy you want. In Schuetzen we want maximun accuracy in as little as 100 and as great as 200 yds.
You don't tell us what you mean by long range. I know people who think that 50 yds. is a long ways off.
In the text books long range starts at 800 yds. I don't think you'll ever see a shooter at 800 to 1000 yds. that doesn't have a spirit level sight. Even most of your Shilouette shooters will have them on their guns, and anybody serious about a good score in Schuetzen will also, altho many, including myself would rather use a "sled" as it is even more accurate than a spirit level sight.
Of course many good scores have been made without a spirit level sight, but it is so much easier keeping things "squared" up that it's foolish not to use one if you can afford it.
PETE