I measure tip to tip.
But, you should also be aware that there are different tip lengths for some brands of the interchangeable needle sets. For example, I have a set of 4" tips and a set of 5" tips from ChiaoGoo.
With the 4" tips and one of their 8" cables, it measures 16" tip to tip, but 18.5" with the 5" tips and the same cable.
Oh yes the tips can make a difference, I have mainly 4" tips....but have a few 5" for a small item it can make a big difference