If you're running a level-appropriate canned module, do you run the combats as written or do you tweak them on the fly?
I don't run canned modules for my home game, but for public games? I adjust encounters for the group. Depending on if it's people I've run for or even after an encounter I have options for adjusting, not just numbers but also tactics. In general I'm better at tactics than most so it's a matter of getting a read on what people like.
Why, though? If one side or the other is getting hammered then so be it - just let it happen.
I don't get to game nearly as often as I would like so I want the combats to be tough but fair. I also don't want to TPK the party. Although they were forced to surrender a couple sessions ago or it would have been a TPK.
You could run the same tough combat ten times and get ten very different outcomes depending on a host of factors not least of which are dice. For me, if I'm not willing to let all ten of those outcomes occur I'm doing it wrong.
Ideally I do any tweaking (or, if it's a homebrew, have it fully written) before I even know which specific characters the players will have in it. At some point, though, I say to myself "that's it" and lock it in.
I don't adjust things very often, if I do its before combat starts and usually because I goofed and realize it after I do one last read of the monsters. I don't think it matters when I adjust, and I don't really change expected difficulty. Sometimes I realize the difficulty will be different from what I had expected.
Probably wouldn't be an issue if I spent more time on prep or ran more linear games.
If the players then throw me a curveball and go off mission or do something unexpected then of course I have to wing it, but even there if I dream up something that's too easy or too hard it is what it is and they have to deal with it.
Too easy is boring, too hard and I'll want to make it obvious and give them an escape route(s)