I personally agree with everything
@KingGrump says, but I think there is more to consider. I am in the Redster CS130, which I would guess is slightly softer than your STI boot Amy, although are you in the 130 or 110? Anyway, my initial runs in moguls were not good due to boot stiffness and having them really tight around my shins/calves. Just loosening the two calf buckles (maybe just the top one and the power strap) to give me a little play really helped. The boots were restricting my natural movements. I am quite comfortable in bumps, so I knew it was the arrow, not the Indian. So lesson 1. don't restrict movement too much when in stiff boots. There is a video on the web about skiing in 150 flex boots, and he advises what I did. (loosen fit around calf to allow some play) When I got in powder, found my COM and stance to be funky. I couldn't shift weight from toes to heel very well without kicking my butt out and feeling weird. Changed forward lean from 18 deg to 16 deg and voila, everything felt good again and butt came back in, legs said "thank you" as well. At some point I also cut the saddle of my boots to soften them a bit as well, probably dropped 10 in stiffness. 140 to 130. All is happy and working well with my skiing technique(s). The 18 deg lean felt great on groomers, especially with carving skis. As KG says, in powder the snow doesn't push back the same, so adjustments are needed. I was doing 3D turns in powder, but it wasn't enough to counteract my boots putting too much force on my tips, unless I shifted the load to my thighs, which made them start complaining after a few runs. All is back in balance now with some tweaks.
In terms of flex stiffness, I think for most people, there is a range that will work for them. But add stiffness, forward lean, and reduced cuff play from snug boots and it can all add up to restricting the movements that are needed. It can also be, as KG suggests, not doing the proper movements for the snow conditions (the Indian) which is where a knowledgeable observer really helps you to sort it out. Back to flex stiffness, I tend to just bounce or flex my boots in the store (or at home) and make a judgement. I know what feels good, so I soften till it feels right (by grinding the saddle). I have never been in too soft a boot, that screams at me right away. They do stiffen with temp, so sometimes what feels right in the store gets too stiff in the cold, but I wait till spring to make any grinding judgements. (don't want them too soft when spring comes)
My CS are the first boot I have owned where I can over tighten the cuff, or restrict my shin/ankle flex too much. All previous boots had play there not matter how tight. Its a new "adjustment" I have to pay attention to now. And its not just cutting off blood flow (did that and feet got really cold) its actually a combination of powerstrap tightness and top buckle tightness that dictates my ankle flex allowed by the boot (without even flexing the shell). I need a little play in the fit (maybe more now that the shell is stiff and I can't flex it quite as much as previous boots) or I can't get the max flex point that I need for my body mechanics.
Hope this is understandable and it helps.