Initially I found riding a loaded bike while standing was awkward and flexed the frame a lot. In relatively short order I adjusted to it and no longer found it to be a problem. The adjustment just seemed to happen with no conscious effort so I am not exactly sure what adjustments I made.

Interestingly enough after a couple months on the road, riding an unloaded bike while standing took a bit of readjustment, but that was a matter of minutes as opposed to a longer adjustment in the first place.