[MaxGunz]

Keep The Ball centered to keep your plane pointing where it's going.

Ball not centered means you are flying in a slip or skid condition. Stalling in a slip or skid is how to get into a spin, especially when at more than 1G which is most turns (you could turn while descending and not be > 1 G) and with enough > 1 G is an accelerated stall (under loaded conditions) that with slip makes for a hard, fast spin.

Actually in a turn you might want The Ball slightly IIRC inside but centered works and it's far easier to remember.

When you're flying straight and trying to shoot, flying in slip will put your shots off to one side of the pipper. If you can use that then fine.

Anybody that says not to rudder in turns is either a fool or trying to get easy targets. It's much like people who tell you to fly slow and use flaps a lot.

Lastly, flying with 2 or 3 degrees of slip to fool someone targeting you is a valid historic tactic to get the attacker or potential attacker to perceive you as turning just by the attitude of your plane and miss the first shot which hopefully you see. Look for BoB videos of a pilot named Bob Doe talking about that. There's others but I remember his name.

If you have any doubts, go and test with mind conscious of what you're checking. Don't just think about it or talk and be fooled. TEST. CHECK.

PS
Spitfires have Turn and Slip needle gauges way down on the lower right front panel. They're hard to check in the game and still see where you're going. I don't remember any but British planes without The Ball.

Learning to rudder right is a matter of practice on a per-plane basis. Practice just flying with your view down enough to see The Ball and in time you won't have to check it much the same as you don't have to watch your speedometer every second when you drive.