It varies a bit with discipline. Heels down is always desirable, but not always possible. Depends on your build and flexibility. If my heels are under me, then they probably will be very close to level - and that ain't going to change anytime soon. I think dressage is more tolerant of level heels, but I don't do dressage so that is just an impression.
Toes in/out: depends. Generally speaking, toes forward to maybe 30 deg out is pretty desirable for most disciplines. I've read jumpers tend to allow toes out some, and dressage more toes front, but I don't do either discipline so that is just what I've read.
As a guy who started riding at 50 (after 40 years of daily jogging), I'll make the plea that toes go forward consistent with a relaxed leg
: I want toes forward, but not if it tenses up my leg. Tight leg muscles make me bounce around and lose balance.
I thought I was doing better, but I noticed yesterday that my gelding is getting so sensitive to leg cues that he was interpreting what was, for me, very mild tension in my thighs as a cue to speed up. Today I just pointed my toes out more to get my thigh as relaxed as possible, and he responded by staying consistent at whatever speed I asked of him. And that was in a western saddle, so the idea that a western saddle doesn't allow a horse to feel cues is a bit silly.
One of my pet peeves is the idea that everyone needs to ride the same. There are a lot of trade-offs in riding, and what works for one rider in one saddle on one horse may not work for someone else. I have very tight legs and hips, which is great for running. I've jogged for 40 years without any injury, and I started with leather street shoes. But the tightness that helps protect me from injuries in running is a big disadvantage in riding - but that is me, and I can't change my body overnight. Maybe never, in some ways.
If you look at pictures of cowboys from 100 years ago, or many military riders, you'll see legs forward. That is frowned on now, but the folks doing it covered far more miles on horseback than most modern riders can imagine. The important thing is to learn WHY things work or don't work, and apply it to your situation. And obviously, if you compete in a judged competition, then that competition will tell you what is expected. A cowboy cutting animals out from the herd on the LS range
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