I just type. My penmanship sucks. 
No crossed 7, open 4, and I usually put a dot in the center of the zero. Don’t know why I do that. When I worked as a telephone operator, we were taught to say the numeral as zero and the letter as oh: and were were never to say eight-hundred, but eight-zero-zero. QA would get us if we didn’t.
Crossed 7s
Crossed Zs
Uncrossed 0s
Closed 4s
Top-serifed 1s
Crossed 7, with a serif on the top left, too.
Usually don’t cross my “Z” unless I’m being fancy.
Uncrossed 0, unless there’s an O I need to distinguish it from. Even then, I usually stick to a dot inside, rather than a line. “∅” is an empty set.
Open 4.
Serif on top and bottom of a 1.
Open four.
Only cross a zero when I need to distinguish it from an O
always cross zed
one is always vertical and seven is always slanted.
I used to cross sevens, but stopped when I realized that mainland Chinese don’t understand what the hell it is. At work I am sometimes involved with Chinese and money. I have seen Taiwanese Chinese cross their sevens.
Maybe I’m insane.
I use crossed sevens. When I was about 15, I trained myself, for no specific reason, to do so. Prior to that, non-crossed.
Also, at about 25, I trained myself, also for no reason other than that I had never seen it before, to make eights with two separate circles instead of one single stroke.
Joe
I cross my sevens when I’m writing Norwegian, and do not cross them when writing English. Ah, the bilingual brain…
What’s a flip tail? You mean the curly upwards thing towards the right? I do that.
Anyway
Flagged and based 1’s, uncrossed 7’s and z’s, open 4’s, figure 8s. I trained my hand to make 0’s and O’s sufficiently different (my 0s are narrow, I can write long strings of random O0o’s and people can tell what’s what usually) so no crosses on zero.
Higher math and physics, which made me use “i” and “t” (for imaginaries and time usually) a lot looked too much like the + sign and a 1 (I do flag and base my ones, but if I’m going fast on a test I may forget one somewhere) so I learned to hook my i’s and t’s to make it easier, no one else I know does this.
Crossed 7s to distinguish them from 1s. My Gs and 6s are interchangeable.
I was under the impression that crossing the 7’s was a military thing? I remember many teachers/co-workers that asked me if my dad (when I was younger) or myself had been in the military.
I thought 7s were crossed to distinguish them from 1s for scientists and mathmeticians with messy handwriting. That’s why I cross them, anyway.
My 1s are straight lines, my 4s are open, my 8s are figure-8s (not snowmen), my Qs are always curly and so are my Ys and Gs. I also occasionally cross my Zs. 0s are uncrossed.
I hook my i’s and t’s when I’m doing math as well.
I started crossing my 7’s in seventh grade math after I confused a 7 with a 1. I never cross my 0’s or Z’s and my fours are always open.
With a cross. I started last summer when I was taking a lot of measurements and writing them down by hand. I cross my Zs too. When I’m being careful a 4 will have an open top. I might try crossing my zeros, but I use both phi and theta all the freaking time. I think that would cause more confusion that it could prevent.
I’m a New Zealander. 7 is crossed, 4 is open, z is occasionally crossed, 0 is only crossed if I’m writing down a password or something where I don’t want to confuse 0 with O.
Crossing the 7 means I won’t confuse it with a 1. I have yet to find an easy way to make my 5’s not look like S’s when I’m writing something in a hurry 
I’ve got a bunch of those - the series was called “Alfred Hitchcock Presents the Three Investigators.” At some point Hitchcock stopped being associated with the series and it became just “The Three Investigators.” I really liked those when I was a kid because they usually took place in California, where I live.
As for the OP, I cross 7s and Zs. I also use open-top 4s and will cross a zero if it is anywhere near an O. I’m left-handed, so my handwriting sucks. I print everything and almost always use block capitals.
I use the 7 with a slash if it’s a group of numbers that may be confused. Otherwise I don’t
I do the line through the Z if it may be confused by 2 also