What should and shouldn't go on the main bus?

I am new to Factorio and I am trying to improve my base creation style.

My current style is to build a big bus that starts with 4 lanes of iron plates, 2 lanes of cooper plates, one lane of coal and one lane of stone bricks. Total = 8 lanes.

From there, I will increments the bus with one or more new lanes for every single component I would ever need.

For example, from those 8 initial lanes I would add 2 lanes for Iron gear wheels, 2 lanes for Cooper cables. Total = 12 lanes. And would only grow from there for every new component.

I was looking at some blueprints in factorioprints for inspiration and what struck me is that in a lot of the designs components like Iron gear wheels and Cooper cable are produced on demand in the blueprints instead of coming from a lane from the main bus.

That why I ask, what I should have in the main bus besides base materials like Iron Plates and Cooper Plates and what I should produce on demand?