More often then not I set something up by eye/hand a bit obtuse, and tack the inside corner letting the weld pull it closed and checking it with a square before tacking up the other corners then welding out. These fab tables, squares, alignment jigs etc have their place, and within that place the save a boat load of time, but for one offs and stuff, you can have more time involved in setting up all these square and jigs than just welding it. And you could STILL have a pretzle when done if you don't know what you're doing.
They're pretty cool though, and if I had a large shop and unlimited money I'd have all of it, but I have neither of those things