Some (or most, if not all) tube benders are not very precision manufactured. So yes, leveling it as best as possible will help somewhat. I feel the best time to check level is when your tube is in the die, and under tention from you pulling on the handle, but NOT bent yet, put a level on the die and see how level it is under load. This is when you actually want it level as much as possible. Put the level in both planes @ 90 degree's. One in the direction of the length of the tube before the bend, then @ 90 in the direction of the bend, still on top of the die. You want it to sweep level as the bend happens.
Now once your first bend is done, and you know you die is as close to level as your going to get it (hopefully pretty level), then when you go to make the 2nd bend in the tube, get it ligned up and ready to bend, have your angle finder on the already bend leg and check for 0 degree, And check your die again for level. If both are pretty damn close, thats about as good as your going to get in my opinion.
Now you have a variable for the second bend that WILL most likely effect your zero degree or level on your die. With the first bend, you have "X" amount of tube hanging out of the bender. This should be supported level with the length of tube being bent around the die, or an center of the die. Otherwise this weight hanging out in air will leverage you die and it will not be level. This goes for pretty much any tube your going to bend. Unless it's just a single bend in a piece of tube.
As a peice of tube gets aditional bends in it, it effects how, and what direction the die is leveraged out of level, which will effect your bending a tube in a single plane. Make sense? It depends on where all the sag weight is at in a 360 degree diameter around the die. Your material should be supported at all times if possible, preferable before and after the bend if all the bends are in the same zero offset plane. Like you were trying to bring the ends of the tube together. This is where an extra set of helping hands is great. Each time you make a bend, and more weight is swung around the die, you will more then likely find the level is out in a different direction as the weight of the tube swings and more weight from sticking out from the die it gets. Understand?
As far as bend angle, I bent to as close to 1/4 of a degree as possible. Manually it takes two people. I would bend and have my son watch the degree wheel. I showed him how to split the degree lines into 1/4's by eye. I can say this was a HUGE factor in my cage coming out as symetrical as it is, which it is really symetrical. I hope some of this info helps. It's freaking irritating to bend a tube in a single plane only to find out it has some degree of twist or offset to it.
And once it's bent, it's damn difficult to get the twist out of it. Takes a lot of leverage and the tube must be clamped to something heavy enough to not lift or move when trying to get a slight twist out of a bent peice of tube. I haven't had to do this for any tube in my cage but I started a tube rear bumper a long time ago and it has a slight twist I need to fix. Not worth bending a new piece of tube up. I'll just get the twist out of this piece. It also take space. You need to use long solid bar or tube with thicker wall then the piece with the twist and stick inside or over the tube, then you need to leverage the hell out of it with this long cheater bar, so space to get a twist out is important too.
If a piece of tube has a really bad twist it may rotational kink as you leverage it. I'd only try and get a twist out if it's just slight. For instance if it's a B piller you are bending up and the top horizontal portion of tube is flat and one vertical leg is flat, but the other leg is lifted off the ground or whatever flat surface you're laying it on, and it's about 1 inch off the ground, then I'd flex it to fix it. Too much more and it could kink. Good Luck.
