I tend to assess the risks and the likelihood of going in. On a warm summer's day on the canal I probably won't wear one, but on the same canal in the depths of winter I probably would- increased likelihood and severity of falling in.
Similarly, having just come up the Thames, I wore one from Brentford up as far as Reading, but would've worn one all the way had it been raining or slippery.
It does need bit of thought, as they can bring hazards of their own, e.g. straps tangling in centre paddle gear when crossing a GU wide lock. But it's just a case of assessing the risks, and if the likelihood or severity of falling in outweighs the smaller risks of wearing a lifejacket, on it goes.
I will probably wear one then, because being tired will mean I'm more likely to make a silly mistake.