The top of the right forearm for a few reasons. Yes, the right forearm specifically as am I indeed left-handed.
1. Lack of Consequences
I do not care about people seeing my fresh cut wounds precisely because there are no actual consequences when it happens. It would be impossible for me to pass an in-person job interview (I know this because every time I've lied on my CV and gotten an interview you could draw a red-dotted-line from the interviewer's eyes straight to my right forearm... and then they've suddenly "found a more suitable candidate" haha) but the funny thing is that I don't even need to pass interviews. Or get a job at all for that matter.
I'm on pretty much the highest disability payment you can get in the UK as a single person (UC w/ LWCRA + enhanced PIP), have neither hobbies nor social life, and live in a rural area so money currently isn't an issue. Before then, I was on Child Disability since around age 12 - so before I started blatantly cutting myself - which pretty much guarantees lifelong access to disability money as an adult here because childhood mental disorders which continue into adulthood are de facto chronic due to NHS incompetence.... and the DWP is much more likely to believe you if your (respectable, employed) parents said the same things about you a decade ago.
I've met people (funnily enough, a lot of them work for the NHS) who have had to switch cutting locations in order to keep their jobs and they just seem absolutely miserable. Being able to cut myself on my own terms is one of the reasons why I'm not having back-to-back CTB attempts; why kill yourself when you can just grab a blade and get high?
2. Visual Communication/Identification
My plainly visible cuts - which, by the way, aren't the neat "tiger stripes" but rather what you'd expect from somebody who mostly loses control (and cuts specifically as a way of letting loose and not worrying about the consequences) as soon as the blade touches the skin - tend to let other
people know immediately, before I even open my mouth, that I have some quite severe mental health problems and am obviously a danger to myself. Nobody I meet is surprised to learn that I'm suicidal.
This is all my deasign. Having a way of showing people via my appearance that I have problems is a fantastic way of preventing conflict. People for whom severe mental issues are a dealbreaker can literally just not speak to me at all instead of getting to know me, finding out I have problems, and then having cut me off. It's a win-win situation.
3. Ergonomics & Comfort
Honestly, this is the primary reason. I have complete control over my forearms. Try moving your own forearm around: you can arrange it in all sorts of positions and angles while still having full visibility of the forearm. This allows me to cut anywhere, anytime, and with any sharp object I can find; when you've been at it for over a decade, choosing the best angle for your current situation just becomes second nature. I can press my palm onto a wall while stood up, I can sit cross-legged with my arm on my lap, and I can even place my hand under my chin so that I can "play the violin".
I also cut for sensory reasons so being able to actually see the blood squirting out, without using a mirror, is *amazing*. Heck, I often bring my forearm to my nose for a sniff of the iron-y smell and can easily drink straight from the source which is definitely an acquired taste - and, yes, my blood is indeed sweeter after I've eaten cake.
Lastly, it is far easier to bandage the forearm (I think only the legs, especially the shins, are easier to bandage since you have both hands free) than many of the other common cutting locations. Now, I do love free-bleeding all over myself - like I said earlier: it's all about losing control for me - and have done so countless times but the reason I bandage nowadays is because I have some nice clothes and blankets that I'd rather not see bloodied whenever the wound reopens.
Many Thanks,
CumbriaCTB