How important is it to set your cargo tonnage when plotting a route? I used to do mining deliveries to sell stations and never once set my cargo tonnage but seemed to get to my destination without running out of fuel. So is it important and what does it actually do?
It is only for planning. You can see for example if you are going to have enough fuel for a trip (how much cargo produces a dotted line) or more simply, how mych cargo can you load whist still making a single jump for a delivery. Stuff like that.
If you forget to plot after loading cargo, no matter - after leaving the station, if you can't make the first jump due to "excess" weight, you will be told so onscreen. Then, just open the Gamap again and your course will be replotted automatically.