I always used to use plain water as it never blocks the jets / pipes etc. & I've had to unblock loads of infrequently used, (usually rear) washers over the years. It freezes more regularly, but I didn't think there were that many occasions when screen wash would prevent a freeze-up.
When I bought Pug, it had been (expensively) maintained by the dealers, (e.g over £200 for little more than an oil change

), and had a full washer reservoir....
Have to admit I was really impressed by cleaning performance of wash/wipe system, (even on a 100,000 mile pitted windscreen). The combination of 'mist' type washers, screenwash fluid & new wiper blades, worked really well even in the cold snap before xmas, so far it's never frozen.
So I broke my own rule & put screenwash in now, (though not at dealer prices - Asda's finest followed by whatever brand Santa uses)