That one could run and run. In the final analysis you should cut the parts (a)to fit to each other and (b) to be the right size- any line is just there for guidance. Get used to your method of marking and your accuracy of cutting.
Examine the processes and act accordingly eg a)dovetail pins marked from tails the line will be entirely on the good material - so leave the whole line whereas b)marking off one plank against another to produce a second part the same length as the first (though ideally you'd have marked them both at the same time - but hey- this is just an example) the line will be entirely on the scrap material - so cut away 99% of the line.
I think it depends on what you're doing and how you've measured it.
Using a pencil to mark,to cut accurately to length, I would mark so that the outside of the line was the length and cut to that - the very outside tips of the saw teeth against the out side of the line.
If marking with a knife, the inside of the knife cut is where the saw cut will be so it has to be measured accordingly.
When it doesn't matter so much about accuracy, deep scribe marks are fine to cut into the centre of: - Roy Underhill's grease box for eg., it doesn't matter how thick the lid ends up, it only has to fit it's box.
Ideally you split the line. If you are having to remember which side to cut eventually you will get it wrong. Also splitting it means you can use a thicker pencil.
All marks should be centre to centre, to avoid ambiguity. This includes the very rare occasions when you would mark with a knife as distinct from cutting - where the line is exactly on one edge or the other of the cut.