“Don’t let your knees goes past your toes” is a one of the cues heard all over the place when teaching people to move, but as you progress and use different exercises it becomes less and less applicable.
The thing with squats, deadlifts, and any exercise really, is that we’re all different shapes and sizes. The length of our bones, angle of joints and even what feels comfortable is individual and can be quite different for each of us, and is going to have an impact on how we move and perform exercises.
The big compound lifts are the best examples, seeing a wide variety of squats, bench and deadlift setups - the biggest and easiest to see difference being in the sumo vs conventional stance of the deadlift. Each has both hands on the bar, and feet planted firmly on the ground, albeit at radically different widths.
Some will adopt a super wide sumo foot position (take a look at Stefi Cohen and Yury Belkin, feet so wide there’s a danger of flat tootsies), some an ultra conventional close foot stance (look at Pete Rubish and Konstantin Konstantinovs, feet close together, hands outside the knees), and some will sit somewhere in between (like Eddy Coan, one of the greatest lifters of all time).
Where you plant your deadlift feet and where you point your knees will depend largely on how the top of your femur (the acetabulum) sits in the hip joint, how flexible your hips are and how long your femurs/arms are, and most importantly what feels the most comfortable. If you’ve got long femurs and have a shallow hip joint (so the acetabulum has more room to rotate), chances are the sumo will be more comfortable, as the long femurs could leave your butt sticking up in the air, making the start of the lift harder. However, if you prefer doing a conventional stance with feet close together, then deadlift that way. Both will work your lower back and are legal in competition if you fancy it.
**This is why the argument “sumo is cheating” is a bit pointless, because not everyone can deadlift well from a sumo stance (meaning conventional is stronger for them, thus the "cheating" stance leaves them with a weaker lift) and while the hip/knee movement is a little different, the weight still needs to be lifted up from the ground. Don’t get mad because some people are using different techniques that are legal for everyone to use**
Safety is the most important part of finding a technique that suits you best - the last thing you want to do is injure yourself if it’s avoidable. This is where the textbook form and cues come into play to get used to a certain way of moving.
As long as we follow the basic safety protocols of each movement (don’t round or overextend your lower back, keep hands an equal distance apart on the bar so the weight doesn’t fall to one side) we can play about with the rest of the exercise details like stance width.
In squatting for example, we might start out with “don’t let you knees go past your toes”, but as we progress and try out deeper ass-to-grass squats, change our stance width or where the bar sits on our back, we might find our knees inevitably peek out over our toes. Keeping them behind would become a balancing act requiring a core of iron.
Instead, as long as our lower back is safe and joints are all moving comfortably in a controlled fashion, we can do what we want with our knees. The same goes for any lift - as long as we’re moving safely in a controlled fashion, feel free to change the details and characteristics of your setup.
By changing your setup slightly and mixing it up with different foot/hand/bar positions, you can find what feels the most comfortable and natural way to move for you.
Even if you find your one true stance, practicing the others can be a great accessory workout, targeting and emphasising muscles that maybe don’t get hit as hard with your regular position. Think close grip and wide grip bench press - close grip is known to hit your triceps harder, while wide grip hits the chest. Both are utilised in the bench press, and it doesn’t hurt to be stronger in either area.
Another reason to test out different stances is that you might not realise how powerful you are until you try them. Cailer Woolam, who holds the current world record deadlift for his bodyweight, a mighty 421kg (928lbs) in the 99kg (220lb) weight class, used to train exclusively convention deadlifts (feet close together), then one day started using a sumo stance to help train the other muscles. Gradually he found that he was able to pull more weight using a sumo stance, so switched over and now breaks world records with it.
It pays to get out the comfort zone, even just a little.