I wrote it without the dash for most of my childhood, until I found out the dash was a possible way to write it. From then on, nobody ever mistook my 7's for 1's anymore. It was great, since I have really bad handwriting and I'd take all the help I could get not to get things wrong on tests. Zeroes get slashes, too, just for safety.
In my experience the dash is unusual (but not unheard of) in the US. I've heard people refer to it as a German or European 7. I've never seen the German 1 that is like an upside down V used by an American.