Layout is different from schematic.
It depends whether you are using mm or mils (thous).
Some people like 0.25 mm or 10 mil grids for placing components and the lowest possible grid for routing.
Other people use a set of 50, 25 and 5 mils or 0.5 , 0.25 , and 0.025 mm grids
At the end, you’ll find grids comfortable for you.
I don’t believe there is really a “recommended” grid size for PCBNew. The reason 50mil is recommended for Eeschema is because the library components are laid out on a 100mil grid (or sometimes 50mil for older, out-of-date symbols) and Eeschema does not snap wires to the pins, so you will end up with a huge mess if 100mil is not evenly divisible by your grid size.
In PCBNew, it doesn’t matter as much, because tracks will snap to the pads. You can even change your grid size while you’re working on the project, without the disaster that would happen if you did this in Eeschema. Although generally, I’d recommend sticking with either a metric grid or an Imperial grid. If I’m doing a project with a lot of DIP components or 0.1" headers, I’ll pick a grid like 50mil or 25mil. If there isn’t a strong reason to go Imperial, then I’ll often pick a grid like 1mm or 0.5mm.
Actually using any concrete measure for schematic is pretty meaningless before you want to print it in paper. The only purpose of schematic is to be human readable, and it’s like plain text - you can zoom in and out as long as you can read it comfortably. Even when printed it could be zoomed one way or another.
There have been developer discussion about getting rid of hard coded concrete units in eeshcema, using arbitrary non-measurable units instead, so that it could be zoomed at will without thinking about mm or inches. It could then be zoomed and fitted for printing and one internal unit would become so and so many inches/mm.
I grew up with a “God-given” PCB grid of 2.54mm but I think todays trend is to use no grid at all. Put components where they fit best and let the router snap to the anchor points.
And I am slowly (!) adopting this kind of strategy.
Nevertheless you can switch off snapping in the settings menu and you can switch of DRC rules enforcement.
Of course you might get invisible ratsnest remains - its up to you to insure connections.
It’s not possible to not use a grid. You can of course select so fine a grid it’s almost stepless. But what does that have to do with DRC? No sane person would ignore DRC with anything but the most simple boards.
@Fleetz: I use 0402 and comparable component sizes, often with 0.2/0.2 track/space width. We need tight spacing and I have found that 0.05mm is often the smallest needed grid. But in general it’s better to use some multiple of the smallest used value, it means lesser work when moving components and routing. Use the finest grid only in tightest places.
For a hobbyist board with THT components and no space requirements I would recommend much larger grid, like 1mm.
It must be remembered that the layout comes from placing physical components, and the centers or edges of the pads of their footprints don’t necessarily hit the grid whatever grid or location you choose. So it’s conceptually very different from schematic.