hide or embrace self harm scars?

It's what you're comfortable with. Ideally, you would one day recover to the point where you can wear short sleeves and feel okay. But if people asking questions might trigger you into more self harm, I say cover up for now. If people ask, just say you don't want to talk about it, or just tell the truth- you are recovered from self harm.

I say, do what makes you feel comfortable. I tend to hide any (very old!) ones from my parents and other adults as they didn't know when I was self harming and don't know to this day that I did, but around my friends I wear whatever I want, short sleeves normally as they know and I'm not ashamed.
Do what makes you feel comfortable and if people ask (which they shouldn't because that's rude) tell them what you want. Ultimately they probably all know, but you don't have to say anything. It's about how comfortable YOU feel.

If you are no longer doing that, be proud to show them off. It's a huge obstacle you've overcome and you should feel happiness.

Embrace the pain