At the shop, we balance tensions with the zig-zag stitch. If we balance so the top thread is just a "tick" on the back, the straight stitch should also be balanced. That's where stitch quality shows up -- in being able to balance the zig-zag stitch. Those cheap, plastic bobbin case machines are a lot harder to balance the zig-zag. The "tick" on the back can be so uneven. But I'm learning that people would rather have a cheap machine with bells and whistles than good stitch quality. There's nothing like an oscillating hook for great stitch quality!