should I?

To practice for the run or just for fun?

Why not both？

As this are the speedrunning forums, I'm answering this from a speedrunner perspective: Not dying doesn't help you much at all in this game. You will die and that is fine. Being fast and knowing strats is way more important than not dying as you at least confidently move through a room instead of being scared because you might die and want to take it too safe. That can lose a lot of time. Also if you really wanna do that you would have to learn deathless strats but at that point why not just learn fast strats that can be very consistent as well even if they have a small risk. I'm not talking about copying world record level strats here of course but having a fast strat for the rooms beats playing it safe and slow without dying most often than not. Even the any% world record has some unintentional deaths

Just to add to this, i've seen some new runners have the idea that being "good at not dying" is useful. Not dying in celeste speedruns is definitely fast, but I wanna point out that there's basically no such thing as being "good at not dying" in general. you can really only be good/bad at individual strats and tech. So going for golden berries and deathless runs is a cool challenge but it's really unrelated to speedrun practice. you might be great at a set of strats that gets you through chapters deathlessly but that ultimately has almost no bearing on how good you'll be at the speedrun strats you choose to learn afterward.

i feel like it could still be useful as you should get a big confidence boost whilst playing thinking "ive beaten this game deathless, I can do anything"

definitely agree

The temporary confidence boost that BlueMushroom64 spoke of might soon go to shambles if you want to speedrun the game afterwards, as you will have to relearn all of your safety-oriented approach. Confidence or not, speedrunning is for a great deal based on muscle memory of specific strats. In my case this confidence boost would soon turn into high expectations and disappointment, since deathless and going fast are vastly different.

On the other hand, if you think a deathless run is fun, go for it. Having fun makes learning a lot easier. Just don't expect your deathless experience to translate very well to anything different than the exact thing that you practiced.

The game has a built-in feature for playing deathless in the form of golden strawberries. I found it helpful for certain stages to get a fast speedrun time while forcing myself to play consistently and deathless. I think my 7B IL PB is still my golden berry attempt. Do what's reasonable for you as an individual.

Deathless runs are challenge runs, and while similar to speedrunning in many ways, they will not necessarily help with each other. Both will help you improve in the game, which will in turn help with both speedrunning and challenge running, however being good at one does not make you good at the other. Doing a deathless run to improve at the game in general or simply for fun makes perfect sense, doing it to specificially to make you better at speedrunning is pointless. If you want to improve at speedrunning without grinding speedrunning, I'd reccomend playing some difficult mods which can be found at https://gamebanana.com/games/6460. Difficult mods will help you gain consistency with some tech as well as get better at timing certain things and lining things up, which is why I find them very helpful. They are much more affective than deathless runs in this area, as the skills required are actually more similar, although challenge running can teach you things like patience and nerve control, which are helpful in their own way. But obviously the best way to improve at speedrunning is just to speedrun, although I know it's nice to have a change of scenery while also still improving at speedrunning, which is where these other things are good. Most important thing is, do whatever you find most enjoyable!

i just fucking died near the end of farewell

That hurts to see