I won't say I'll always swatch from now on, because let's face it, if I've not learned by now, I won't ever learn.
They would be a really good idea though. Project asked for 18st/28 rows per 4", and I get 24st/32 rows. Something went badly wrong. Ripped out many many hours work and started again.
Good news is I now get 20st/28 rows which is acceptable to me, if I dropped down any more I'd start being off on my row count.
I did not cry and I did not panic, but I may have swore a few times.
Like I said though, I won't learn, but I wish I would!