This is an issue that tends to stir a lot of hot, interesting, thought-provoking, controversial debates.
For now, I mix exclusively in the box . . . as I don't trust the quality of my D/A converters. This approach may change if I someday discover a noticeable enough difference in sound quality through analog summing.
Mixing/summing analog will unavoidably entail two more generations of digital conversion -- once for each track, bus, submix or what have you (however it is you tend to work) on it's way out of your DAW. And yet another back in for the final mix. If it's being mastered, there is another probability / opportunity it will be converted yet again, depending on how you or the mastering engineer work.
So what it really comes down to is which you feel is the lesser of two evils -- the added conversion steps, or the added rounding/math errors during the software-aided mixdown process. I am not Ethan Winer,

so I know very little about what is really going on, technically, with all of those 1's and 0's.
. . . But knowing just enough to be dangerous, I guess,

logically, I figure with all of those added generations of conversions, there might be a mathematically greater chance of sound degredation -- particularly from a cumulative standpoint -- by going the "outside" route. Also, my ears aren't telling me there's a big enough difference to justify it for now, either, so that's where I stand -- rather shakily -- for now.