People will probably lose their shit reading this title.

I use rulers with fountain pens all the time, but not to draw lines. Most of my use of rulers is actually to make it quicker to divide things into sections.

When I use a Rhodia Dotpad for my hit-lists I always split it into three sections if I use it vertically, then I just find a ruler to quickly put a mark on the dot closest to 5cm and 10cm on my ruler, then I put it aways draw 2 lines. Takes me less than 30 seconds, would have spent at least 3 times that doing it in any other way.

If I’m actually putting a pen against the ruler, especially since all of mine are metal, I would certainly use a gel pen or something.