These days, many devices have multi-band wireless hardware. Dual-band is very common, tri-band, less so, but even quad-band is not exactly unheard of.
In the halcyon days of Wireless-N, devices came from the factory with separate SSIDs for 2.4- and 5-GHz networks. Some time during the Wireless-AC era, this changed, and devices started to come out with a single SSID common for both bands.
So I wonder, what is the preferred approach with OpenWrt? Is it a good idea to have the same SSID / key pair assigned to all wireless networks a device services? Is it a bad one? Or does it depend on the underlying hardware or the applicable wireless standard(s)?
I highly recommend a single SSID (and of course the same encryption type and pass phrase) across both (or all 3) bands. This allows the client device to select the band that is best based on the internal logic of those client devices based on the instantaneous conditions.
Personally, I have a base name for my APs and then add an additional identifier for the unit and the band to the name. Yes that makes roaming a tad less smooth, and I need to add each instance one by one to all end devices, but I get actionable problem reports from my users that tell me where an issue might be located. I am not claiming this to be ideal for anybody else though...
Oh, I also change the encryption between devices (to keep older encryption only on the AP serving older devices incompatible with wpa3). So while I agree with @psherman's rationale, I mostly ended up violating his recommendations for "reasons".
P.S.: I am not sure all my devices have competent band selection methods, so having mostly to do that manually is a mixed blessing, more work but also more consistency...
It depends on the client devices. Some of them select the right band correctly and all is well, but if you have a device that is a bit dumb and chooses on the basis of simply noise not bandwidth it will have a tendency to switch to 2.4Ghz and speed will drop dramatically. The router isn't in control of this so using separate SSIDs becomes a necessity to get the client devices to behave well.
My hope is with wifi 7 and MLO we might finally see the end of this problem.
In my experience most client devices are pretty terrible at roaming between access points and choosing the best frequency band.
But you can exert some control from the router / access point side. I use separate access points that don't run OpenWrt, but I've had a fair bit of success using a single SSID for all access points and bands, while carefully choosing transmit power for the different bands to ensure that devices favour the correct ones.
Some trial and error was involved, and devices still sometimes manage to pick the wrong band or get "stuck" to the wrong access point, but generally it works pretty well.