In my experience, you can get away with a lot up to 10-25MHz. Mostly just proper board stackup (the bare minimum ensurance that all signal traces are on a layer

In my experience, you can get away with a lot up to 10-25MHz. Mostly just proper board stackup (the bare minimum ensurance that all signal traces are on a layer that's next to a solid ground plane layer, with no slots or cutouts under the traces). Things like trace length, reflections, power dips when the bus is all 1's, electro and/or magnetic crosstalk, etc. become an issue at around 50MHz, although I've never gone that high in a hobbyist design...