Experience has taught me - the hard way - that hiking boots should generally be fitted larger than a typical shoe size. I currently wear a 10 or 10.5 dress shoe or trainer, but my hiking boots are a full size larger for the right downhill comfort balance. That's the tricky and very individual bit - finding the boot size that allows your toes enough room on the sustained downhills, but not so large that your feet slide back and forth, creating hot spots.
For myself, I've found that any small space variance can be made up by the right balance of sock/liner thickness and lacing style. I lace normally going any direction but downhill. Before a sustained downhill I re-lace, bringing the laces over the tops of the uppermost speed-lacers from the inside, then tying tightly underneath the lacer. This seems to keep my foot seated in the heel of the boot and prevents the dreaded toe-jam.
Like DUG, extra socks and liners are my 11th essential. Change early, change often. On a long hike the outside of my pack looks like a tenement, with all the flopping foot garments. Lots of air and as little moisture as possible. Hiking in the humid South taught me that lesson long ago. Since I've done these things religiously, I've had zero blisters or accordion-toe problems.
Feet and footwear are such an individual thing, though. If the most common foot/boot remedies aren't working, a trip to the podiatrist may be the least painful/expensive way to go. Those feet are everything to a hiker - not getting it solved is the only unacceptable course.
The other thought I had, Joe, is that I know you typically dayhike with a lighter pack, but have been carrying a lot more weight on your training hikes the past few weeks to prepare for our overnight trip up Whitney next week. That extra weight can subtley change the dynamics of your feet and boots. Has this issue cropped back up just since you added the additional weight?
