Depends on the friend. I've got three. When the first realized I was serious we had a lengthy discussion about how much it would impact them (after I said that death happens, and they'd eventually learn to live without me), and every time I bring it up (uncommon) it's just another "I'd miss you, couldn't replace you." Second one just generally struggles to hold serious conversations, not with malicious intent, but they're even dodgy about discussion of my self harm, they gave the usual "I'd be sad" but it was pretty open and shut. My third friend though definitely put into perspective how it'd hit him, said it would destroy him (which i believe), but said "at the end of the day that's a choice you will, and have to, make all on your own."
You should bring it up if you want to get perspective or if you want them to be prepared. I figured the shock would ruin everyone around me more than just dying. Regardless, don't forget that once you say it you can't take it back; they'll know for good, and since I'll be around for a while it gives them the full picture.