I use them or an electronic version. "They" say the human eye will only start detecting cant around 3 degrees plus if there's a straight line downrange like the edge of a target board I tend to perceive it as level and use it as a guide and it could be way off. It would depend on the type of shooting though. Settled prone or bench, yes why not. 22 PRS, I've enough to worry about besides throwing cant in to the mix and the targets are forgiving enough anyway that a bit of cant isn't the reason I'm missing.

I don't have one, but have considered if it is a source of error for me as by foreend is rounded and doesn't sit flat on a rest.

Would also be interested if the difference it makes for point of impact, tried working it out but got a bit complex

Just my 2c worth.

A slight cant in shooting is not a big issue, several high-end Olympic shooters shoot with a slight cant. I see lads with level bubbles but most actually never look at them.

What matters is consistently – if you shoot with a slight cant every time and it’s exactly the same every time, every shot then that’s fine, no big issues.

On the other hand, if your set-up is different for every shot then you need to address this, practice, practice and more practice leads to consistency.