I generally put one in if there's a row that's especially complicated coming up. If the pattern tends to flow pretty easily, I put one in 'after a while.' That is, when I notice that there hasn't been one for a while. The problem with that is that I don't always remember what row I was on if I have to go back to the lifeline. So it's probably best to mark your pattern, or put one in after a certain row.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry