[jacquesm]

Yes, the EMI thing is real but not at typical hobbyist frequency ranges. But the mechanical aspects are far, far more important and right angles are simply a bad idea. Ideally the lines are smoothly flowing (like say at the bottom of the old KIM boards), but that's not how auto routers place the traces. 45 degree angles in succession are a good enough compromise. They're mechanically reasonably strong, they don't delaminate and can be easily placed and used for bus patterns with closely spaced traces that will reliably etch without the outside being eaten up and the inside being 'too late'.

[joshspankit]

What frequency ranges does it start to become an issue? Cheap controllers can be on in the hundreds of MHz now.

[petsfed]

>The limiting factor that will determine whether any resonances are excited is the size of the square region in a right-angle PCB trace. In particular, the lateral size of the region will be approximately equal to the quarter wavelength of the lowest order resonance, so this gives you a good baseline for estimating the fundamental resonance frequencies. The remaining harmonics will be approximately odd multiples of the fundamental frequency. If we allow a very generous trace width of 30 mils with an effective dielectric constant of 3.5, the lowest order frequency is 112 GHz! If we take this as a knee frequency for a digital signal, this is equivalent to a 3 ps rise time, which is well-below that for commercially available digital components. [0]

That whole article addresses the myths surrounding right angle traces pretty effectively.

0. https://www.nwengineeringllc.com/article/right-angle-pcb-tra...