Totally depends on how sensitive you are to a delay really. I can personally play any instrument just fine at the lowest buffersize which gives a roundtrip of ~4ms on my system, and if I’m forced to higher buffer sizes I monitor through my analog desk which is even faster than direct monitoring 
. If your rig can’t achieve low latency then I would personally always prefer mono monitoring. It’s just for when doing the recording anyway, as long as you can clearly hear what you’re playing it’s not a big deal.
I don’t know what is most common, I imagine many interfaces do offer stereo direct monitoring but I clearly don’t know much about that as I assumed the UR22 did as well . If you’re not relying on software fx during monitoring I think the best option is analog monitoring like I do on my mixer, but that’s not really an option for you without another investment. I’d say try both and see which you prefer!