On my bikes with 15mm axle nuts, I keep a stubby combination wrench in the tool pouch under my seat, along with a patch kit, multi-tool, and usually a spare tube. I just buy the shortest, cheapest one I can find. I've considered cutting off the open end, so it's just a very short box end wrench. This would make it easier to pack. Haven't gotten around to that yet.

I have one bike with Phil Wood hubs that use 6mm internal hex bolts ("Allen" bolts). I carry an Allen wrench on it. I almost always have a multi-tool, but some of them are so short, they might not give enough leverage to loosen the axle nuts. A slightly longer stand-alone wrench ensures I won't get stranded.