I do all my planetary imaging with the 3x barlow, regardless of the seeing.
It's more of a critical factor for visual.
If the seeing doesn't hold up to a 3x barlow when imaging, I generally don't bother.
However, this again depends on what sort of image scale you are after?
It also depends on what quaility of image you are happy with.
Most planetary imagers work around the f30-f40 range to produce their images.
There are no hard and fast rules. It's all horses for courses.
You set the benchmark, you make the decision based on what you are hoping to achieve
