In winter I always try to use the concentrated stuff and vary dilution according to the conditions (1 part SW to 3 parts water is supposed to be good to -8 deg C and it's been fine during the recent cold snap) I keep the cans behind my wheelie bin and so far despite nightime temps dropping down to -12 deg c they have remained unfrozen in either neat or diluted to recomended concentrations.
When I bought a case (4 x 5L it worked out at £1.50 per can)
I do add a good squirt of washing up liquid now to reduce the smear effect.
100 mile drive last night (at -5 deg according to the car) from seeing my brothers family and I had no issues with frozen washers BTW headlight washers make a pretty pattern in frozen screenwash - soon as we stopped it started melting and running of the wings so snapped this picture pronto.
Incidently this stuff (supermarket ready to use was utter rubbish) but I had to get something from a garage at the end of a 500 mile round trip as their water pipe was frozen solid - it was nearly twice the price I pay for concentrate and effectively was no better that water
note the rating - 2 deg C
It irritates me so much i turn off my headlights briefly if I have to wash the windscreen at night as it makes such a mess!