I make it like either of those. I think I did the slash thing briefly in grade school but it never stuck.
The only thing I put a slash through is the letter “Z” because I have terrible handwriting and mixing up a Z with a 2 somewhere in the middle of a 2 page diff eq problem was becoming an issue.
Granted, I don’t do a whole lot of multi variable math these days, but the habit is still there.
I occasionally write the slash if it’s very important that the number be read correctly. In such a case I’ll also put a hook at the top and serif at the bottom of any 1’s.
European: learned, as a Boy Scout, that “1” and “7” are easily confused and “-” can make all the difference (especially if you hastily scribble down a note that’s meant to relay vital information, For Want Of A Nail and all that).
This for me too. I find that Europeans write the digit “one” with a more significant top-left piece, which looks more like an American “seven”, so the slash helps distinguish the “seven”.
Back when I scrutineered dance competitions, the same mark could be a 7 if written by an American, a 4 if written by a continental European, and a 1 if written by a Brit. (I think I have the nationalities right that go with the different orthographies.) But even putting a slash on a 7 doesn’t always eliminate all confusion, because some Americans use the horizontal base on a 1, and if they are writing fast without looking at the paper because they are trying to see the dancers, sometimes they end up with a 1 that looks a lot like a crossed 7.
No slash. My ones and sevens are distinct. It isn’t much of a problem from other people but if you point your ones you should slash your sevens. In ye olde days, working with coding sheets and the like, pointed ones and unslashed zeroes were frowned upon. If you got frowned upon three times you got fired.